fix ParseCloudFunction execute and executeObjectFunction headers fail

[hellokidder]

hi ! brother,I'm come again !
today,When I used ParseCloudFunction, I encountered a problem, the headers parameter failed,and I fixed it.
code:

    final ParseCloudFunction parseCloud = ParseCloudFunction('functionName');
    final Map<String, String> params = <String, String>{
      'objectId': '5b625c781235522020dbbad9c',
    };
    final Map<String, String> headers = <String, String>{
      "content-type": 'application/json'
    };
    final ParseResponse result =
        await parseCloud.execute(parameters: params,headers: headers);

from
request.headers :{content-type: text/plain; charset=utf-8}
now
request.headers :{content-type: application/json; charset=utf-8}
and the function it's work

At last :look forward to your reply

[phillwiggins]

Amazing, great work!