Configure header elements of a request

[sebastianneubert]

It should be possible to add special header data to the request object.

For example it should be possible to set the content_type of a request to application-xml.

# testsuite.yml
example:
    headers:
      - {Content-Type: 'application/xml'}
    Pages:
     - { url: /api/rest/users.json, post: { email: testuser@test.local, nickname: test, name: tester } }

The request to /api/rest/users.json should have the header:

'Content-Type: application/xml; charset=utf-8'