make contourcarpet w/ levels contours work w/ closeBoundaries

... with similar `edgepaths.length vs starts.length` logic than
    previous commit.

    Note that this patch may also fix un-reported bug for contour
    traces with level contours. I haven't been able to spot any
    while working on this patch.

src/traces/contour/close_boundaries.js

@@ -23,7 +23,8 @@ module.exports = function(pathinfo, contours) {
 
             for(i = 0; i < pathinfo.length; i++) {
                 var pi = pathinfo[i];
-                pi.prefixBoundary = !pi.edgepaths.length && edgeVal2 > pi.level;
+                pi.prefixBoundary = !pi.edgepaths.length &&
+                    (edgeVal2 > pi.level || pi.starts.length && edgeVal2 === pi.level);
             }
             break;
         case 'constraint':

src/traces/contourcarpet/plot.js

@@ -330,10 +330,7 @@ function makeFills(trace, plotgroup, xa, ya, pathinfo, perimeter, ab2p, carpet,
     var hasFills = coloring === 'fill';
 
     // fills prefixBoundary in pathinfo items
-    //
-    // N.B. cheater_contour mock fails if we call closeBoundaries
-    //      on contourcarpet traces that with `levels` contours
-    if(hasFills && trace.contours.type === 'constraint') {
+    if(hasFills) {
         closeBoundaries(pathinfo, trace.contours);
     }