Do not set user-agent header on Flutter Web (parse-community#249)

Author: rostopira

lib/src/network/parse_http_client.dart

@@ -17,7 +17,8 @@ class ParseHTTPClient extends BaseClient {
  /// Overrides the call method for HTTP Client and adds custom headers
  @override
  Future<StreamedResponse> send(BaseRequest request) {
- request.headers[keyHeaderUserAgent] = _userAgent;
+ if (!identical(0, 0.0))
+ request.headers[keyHeaderUserAgent] = _userAgent;
  request.headers[keyHeaderApplicationId] = data.applicationId;
  if ((_sendSessionId == true) &&
  (data.sessionId != null) &&