Pull request for issue #45. Completely tested

This is a pull request that resolves [issue 45](https://github.com/Lusitanian/PHPoAuthLib/issues/45)

OK, had to update AbstractService access token request at line [95](https://github.com/Lusitanian/PHPoAuthLib/blob/4c9447a7e102a96a89f4e96270a337fd7eae0b0f/src/OAuth/OAuth1/Service/AbstractService.php#L95) because a OAuth 1.0a access-token-request does not require a `oauth_callback` parameter.

So, I changed it to use the regular [buildAuthorizationHeaderForAPIRequest](https://github.com/Lusitanian/PHPoAuthLib/blob/4c9447a7e102a96a89f4e96270a337fd7eae0b0f/src/OAuth/OAuth1/Service/AbstractService.php#L183) function instead, because it `unset` 's the `oauth_callback` parameter