Content-Type default should be configurable

Method createResponse() sets the Content-Type to text/html when not present in response.
Content-Type is not a required header, so this module should be able to test it's absence with `->dontSeeHttpHeader("Content-Type")`.

I would suggest to introduce an option like 'defaultContentType' which defaults to the current 'text/html' but can be set to null.

Code link:
https://github.com/Codeception/module-phpbrowser/blob/master/src/Codeception/Lib/Connector/Guzzle.php#L103