Use 'Content-Type' instead of 'content-type'

Really shouldn't matter, but [sometimes it does](sinonjs/sinon#547)

core-ajax.html

@@ -307,7 +307,7 @@
         return header.toLowerCase() === 'content-type';
       });
       if (!hasContentType && this.contentType) {
-        args.headers['content-type'] = this.contentType;
+        args.headers['Content-Type'] = this.contentType;
       }
       if (this.handleAs === 'arraybuffer' || this.handleAs === 'blob' ||
           this.handleAs === 'document') {