Fix for number output bug.

java.lang.Number is currently output without any validation. For all java.* Numbers, this is fine, but for custom Number implementations like Complex or Fraction, the resulting JSON output may be invalid.

For example: If a Fraction implementation defines its' toString method as `return numerator + "/" + denominator`, then the resulting JSON output would be something like this:

```json
{ "fraction" : 1/2 }
```

This is not valid JSON.

This commit verifies that the string representation of the number is close to a JSON formatted number by use of the BigDecimal constructor. If the constructor throws a NumberFormatException, then the string value is instead quoted as a string. The example above would instead output like the following:

```json
{ "fraction" : "1/2" }
```

JSONObject.java

@@ -838,7 +838,7 @@ public static String numberToString(Number number) throws JSONException {
         }
         testValidity(number);
 
-// Shave off trailing zeros and decimal point, if possible.
+        // Shave off trailing zeros and decimal point, if possible.
 
         String string = number.toString();
         if (string.indexOf('.') > 0 && string.indexOf('e') < 0
@@ -1693,7 +1693,18 @@ public static String valueToString(Object value) throws JSONException {
             throw new JSONException("Bad value from toJSONString: " + object);
         }
         if (value instanceof Number) {
-            return numberToString((Number) value);
+            // not all Numbers may match actual JSON Numbers. i.e. Fractions or Complex
+            final String numberAsString = numberToString((Number) value);
+            try {
+                // Use the BigDecimal constructor for it's parser to validate the format.
+                new BigDecimal(numberAsString);
+                // Close enough to a JSON number that we will return it unquoted
+                return numberAsString;
+            } catch (NumberFormatException ex){
+                // The Number value is not a valid JSON number.
+                // Instead we will quote it as a string
+                return quote(numberAsString);
+            }
         }
         if (value instanceof Boolean || value instanceof JSONObject
                 || value instanceof JSONArray) {
@@ -1783,6 +1794,30 @@ static final Writer writeValue(Writer writer, Object value,
             int indentFactor, int indent) throws JSONException, IOException {
         if (value == null || value.equals(null)) {
             writer.write("null");
+        } else if (value instanceof JSONString) {
+            Object o;
+            try {
+                o = ((JSONString) value).toJSONString();
+            } catch (Exception e) {
+                throw new JSONException(e);
+            }
+            writer.write(o != null ? o.toString() : quote(value.toString()));
+        } else if (value instanceof Number) {
+            // not all Numbers may match actual JSON Numbers. i.e. fractions or Imaginary
+            final String numberAsString = numberToString((Number) value);
+            try {
+                // Use the BigDecimal constructor for it's parser to validate the format.
+                @SuppressWarnings("unused")
+                BigDecimal testNum = new BigDecimal(numberAsString);
+                // Close enough to a JSON number that we will use it unquoted
+                writer.write(numberAsString);
+            } catch (NumberFormatException ex){
+                // The Number value is not a valid JSON number.
+                // Instead we will quote it as a string
+                quote(numberAsString, writer);
+            }
+        } else if (value instanceof Boolean) {
+            writer.write(value.toString());
         } else if (value instanceof JSONObject) {
             ((JSONObject) value).write(writer, indentFactor, indent);
         } else if (value instanceof JSONArray) {
@@ -1795,18 +1830,6 @@ static final Writer writeValue(Writer writer, Object value,
             new JSONArray(coll).write(writer, indentFactor, indent);
         } else if (value.getClass().isArray()) {
             new JSONArray(value).write(writer, indentFactor, indent);
-        } else if (value instanceof Number) {
-            writer.write(numberToString((Number) value));
-        } else if (value instanceof Boolean) {
-            writer.write(value.toString());
-        } else if (value instanceof JSONString) {
-            Object o;
-            try {
-                o = ((JSONString) value).toJSONString();
-            } catch (Exception e) {
-                throw new JSONException(e);
-            }
-            writer.write(o != null ? o.toString() : quote(value.toString()));
         } else {
             quote(value.toString(), writer);
         }