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
Added tests for invalid usages of the drivers

These tests cover calling the driver for XPath not matching elements on
the page (case which should not happen in normal usage of Mink given that
the driver is accessed from found elements most of the time) or for XPath
matching an invalid element (trying to check a div for instance).
These tests ensure that a DriverException is thrown rather than doing an
undefined behavior (potentially a fatal error).
Removed the session uniqueId

This uniqueId is not used anywhere in Mink, and the value is not even
guaranteed to be unique. On windows, uniqid does not have enough entropy
to ensure that the testsuite passes consistently for instance.
If the external code needs a way to identify a Session instance uniquely
as a scalar, it is better to use spl_object_hash, which will not change as
long as the Session is not garbage collected (at which point it is
probably the end of the test process).
Select/radio button/checkbox/upload field no longer match by placeholder
Merge pull request #505 from stof/test_invalid_driver_usage

Added tests for invalid usages of the drivers
Merge pull request #507 from aik099/select-with-placeholder-found-fix

Select/radio button/checkbox/upload field no longer match by placeholder
Fixed the phpdoc of the CoreDriver class
Merge pull request #506 from stof/remove_unique_id

Removed the session uniqueId
Added a test for response headers
Added a test covering the resetting of the basic auth
Merge pull request #508 from stof/test_response_headers

Added a test for response headers
Merge pull request #509 from stof/test_reset_basic_auth

Added a test covering the resetting of the basic auth
Make all elements use SelectorsHandler and Driver of their own

1. need to pass SelectorsHandler/Driver in constructor instead of session