message: fix(common): don't guess Content-Type for FormData bodies
  Fixes
diff: @@ -266,9 +266,9 @@ export class HttpRequest<T> {
  if (this.body === null) {
  return null;
  }
  - // FormData instances are URL encoded on the wire.
  + // FormData bodies rely on the browser's content type assignment.
  if (isFormData(this.body)) {
  - return 'multipart/form-data';
  + return null;
  }
  // Blobs usually have their own content type. If it doesn't, then
  // no type can be inferred.
  