Fix CSW HTTP POST Resource URL with body Bug

[Maretius]

The Catalogue Service (CSW) with HTTP POST Resource URL with body - Probe doesn't send a Content-Type and generated a 415 (Unsupported Media Type) Error. It was tested with tcpdump and wireshark and  GeoHealthCheck version 0.8.3. After changing the return value to "{'content-type': self._parameters['content_type']}" of the get_request_headers method the problem was solved.