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.
Improved the conversion of header names to SERVER keys

- AUTHORIZATION is now sent as a normal header rather than being converted
  to PHP_AUTH_DIGEST. This was a weird conversion done for Symfony years
  ago but the AUTHORIZATION header support has been fixed in
  HttpFoundation since then.
- The CONTENT_* headers are not prefixed with HTTP_ to match the behavior
  of PHP.
Merge pull request #53 from stof/fix_request_header

Fix handling of request header