Xiangzhou/fix issue make post request

[xiangzho72]

1. Fix the error when calling make_post_request() by following suggestion from : https://stackoverflow.com/questions/13175253/python-error-type-error-post-data-should-be-bytes-also-user-agent-issue

2. Change iomessage::contextdata and contentdata from type 'bytearray' to 'string', otherwise, user has to use following :
   msg.contentdata = bytearray('hello world'.encode()),
   Rather than the more intuitive way below:
   msg.contentdata = 'hello world'

   my thought is those two fields should be like other fields in iomessage: string when processing, and transformed to bytearray just before sending to wire

3. Updated broken test cases. I have verified them in my local environment.

The change is branched from tag v2.0.1