Fix headers being carried between parts. fixes #63 (#135)

The contents of the headers dict wasn't cleared when a new part started, If the second part didn't overwrite the header value, it would appear in the wrong part.

multipart/multipart.py

@@ -1613,7 +1613,9 @@ def _on_end() -> None:
             is_file = False
 
             def on_part_begin():
-                pass
+                # Reset headers in case this isn't the first part.
+                nonlocal headers
+                headers = {}
 
             def on_part_data(data: bytes, start: int, end: int) -> None:
                 nonlocal writer

tests/test_data/http/mixed_plain_and_base64_encoding.http

@@ -0,0 +1,23 @@
+----boundary
+Content-Type: text/plain; charset="UTF-8"
+Content-Disposition: form-data; name=field1
+Content-Transfer-Encoding: base64
+
+VGVzdCAxMjM=
+----boundary
+Content-Type: text/plain; charset="UTF-8"
+Content-Disposition: form-data; name=field2
+
+This is a test.
+----boundary
+Content-Type: text/plain; charset="UTF-8"
+Content-Disposition: form-data; name=field3
+Content-Transfer-Encoding: base64
+
+VGVzdCBzdHJpbmcuCg==
+----boundary
+Content-Type: text/plain; charset="UTF-8"
+Content-Disposition: form-data; name=field4
+
+This is also a test.
+----boundary--

tests/test_data/http/mixed_plain_and_base64_encoding.yaml

@@ -0,0 +1,18 @@
+boundary: --boundary
+expected:
+  - name: field1
+    type: field
+    data: !!binary |
+      VGVzdCAxMjM=
+  - name: field2
+    type: field
+    data: !!binary |
+      VGhpcyBpcyBhIHRlc3Qu
+  - name: field3
+    type: field
+    data: !!binary |
+      VGVzdCBzdHJpbmcuCg==
+  - name: field4
+    type: field
+    data: !!binary |
+      VGhpcyBpcyBhbHNvIGEgdGVzdC4=