Allow mult-dimension arrays when generating base string for HMAC-SHA1 signature. Fixes #39

[jtsternberg]

With this implementation, multi-dimensional arrays can be passed to League\OAuth1\Client\Server::getHeaders() and be correctly transformed. For prior art, consider the WP REST API - OAuth 1.0a Server plugin. Specifically:

• https://github.com/WP-API/OAuth1/blob/master/lib/class-wp-rest-oauth1.php#L667-L674
• https://github.com/WP-API/OAuth1/blob/master/lib/class-wp-rest-oauth1.php#L706-L753

[stevenmaguire]

There are some code style issues that need to be resolved: https://travis-ci.org/thephpleague/oauth1-client/jobs/108124923

Also, I would like to request to that you provide some test cases that support the changes you are making.

[jtsternberg]

Yes, you're right. Ashamed to admit I didn't read the CONTRIBUTING.md until AFTER submitting.

[bencorlett]

Ditto @stevenmaguire :)

Check out fabpot/php-cs-fixer :)

[jtsternberg]

Closing in favor of #61 which contains tests, and code style issues are addressed.