The 'CannotOverwriteExistingCassetteException' exception now takes two kwargs, cassette and failed requests, in order to get the request(s) in the cassettes with the less differences and put those details in the exception message.
Previously request.headers was a normal dict (albeit with the
request.add_header interface) which meant that some code paths would do
case-sensitive matching, for example remove_post_data_parameters which
tests for 'Content-Type'. This change allows all code paths to get the same
case-insensitive treatment.
Additionally request.headers becomes a property to enforce upgrading it to
a CaseInsensitiveDict even if assigned.
