Corrected the multi-header mishandling (issue #161)

Corrected the multi-header mishandling (issue #161), and disabled two related (now failing) unit tests that are believed to be invalid.

**Note:** This pull request should not be accepted until someone confirms that the related V4 test suite files are indeed invalid (see issue #161).  If/when the V4 test suite is corrected, then the skip added to the `SignatureV4Test.php` file can be discarded.

Let me know if there's anything more I can do to help.

Cheers.