Fix RequestHeaders

[arosslau]

Hey,

This PR fixes setting Headers before sending a request in the request-class.
Actually the Client got instantiated on each Methodcall of a Requestinstance which resulted in setting headers before a request had no effect.

e.g.

$request->setHttpHeader(xxx, yyy); // invokes __call in request which first instaniates a new client
$request->send(xxx); // here you've got a fresh instance of the client, too

[sanpii]

Can you add a test for this bug?

[arosslau]

Hey, Sorry for the late response. Sure, i will rebase to master and add some tests in next few days.

Ps: Is there any Documentaion of how to setup the environment? :-)

[sanpii]

Is there any Documentaion of how to setup the environment?

You can follow instructions from travis configuration: https://github.com/Behatch/contexts/blob/master/.travis.yml#L29-L38