change test for json|xml to use regex, allow spaces in header value

[gmdayley]

This changes the test for content-type(s) to use a regex. This allow for spaces in content-type header to be correctly detected for json or xml types:

application/json;charset=utf-8
application/json; charset=utf-8
-------------------^-----------------

[aacerox]

Hi @gmdayley
You can set a custom content-type for json or xml using the "mimetypes" initialization parameter. Please refer to documentation (https://github.com/aacerox/node-rest-client#options-parameters) and issues #17 and #12

Thanks.