Add default content-type headers to support ES6.0

[ciscorn]

Since ES6.0, all requests must pass the "strict content-type checking".

This PR is based on the following commits on elasticsearch-py:

• elastic/elasticsearch-py@d862c8a
• elastic/elasticsearch-py@073db90

Without this, the following error occurs with ES6.0 and later:

TransportError(406, 'Content-Type header [application/octet-stream] is not supported', {'error': 'Content-Type header [application/octet-stream] is not supported', 'status': 406})

Closes #27 #28

[honzakral]

Thank you for the patch, @ciscorn!