Added the HttpFoundation-based fixture for the error 500 page

Ref https://github.com/Behat/Mink/pull/500
Merge pull request #52 from stof/test_error500

Added the HttpFoundation-based fixture for the error 500 page
Fixed the conversion of header names to server keys to support any header
Refactored the handling of headers

The headers are kept in the driver and passed to the BrowserKit client
when sending the request to avoid altering the state of the client itself.
This allows resetting them properly when resetting the driver.