Use a semicolon as the separator for multipart form-data requests.

pkg/http/lib/src/multipart_request.dart

Index: pkg/http/lib/src/multipart_request.dart
diff --git a/pkg/http/lib/src/multipart_request.dart b/pkg/http/lib/src/multipart_request.dart
index b9adf9cc5c2eeede8165fa9b7c32ba390e281475..3cd0dc405b32d124d9df8929bdbe7e68a02c8873 100644
--- a/pkg/http/lib/src/multipart_request.dart
+++ b/pkg/http/lib/src/multipart_request.dart
@@ -81,7 +81,7 @@ class MultipartRequest extends BaseRequest {
   InputStream finalize() {
     // TODO(nweiz): freeze fields and files
     var boundary = _boundaryString(_BOUNDARY_LENGTH);
-    headers['content-type'] = 'multipart/form-data, boundary="$boundary"';
+    headers['content-type'] = 'multipart/form-data; boundary="$boundary"';
     headers['content-transfer-encoding'] = 'binary';
     super.finalize();