Added a test covering resizeWindow
Added a test to ensure consistent behavior accross drivers for 500

This ensures that drivers don't throw an exception if the page is a 500.
An exception in visit() is allowed if the HTTP request cannot be
performed, but not if the tested app returns a valid error response.
Added a test to ensure that request headers are properly reset
Merge pull request #493 from stof/test_coverage

Added a test covering resizeWindow
Merge pull request #500 from stof/test_error500

Added a test to ensure consistent behavior accross drivers for 500
Merge pull request #502 from stof/test_request_header_reset

Added a test to ensure that request headers are properly reset
Added a test submitting an empty textarea

This covers the issue reported in
https://github.com/Behat/MinkGoutteDriver/issues/10
Merge pull request #501 from stof/test_empty_textarea

Added a test submitting an empty textarea