Fixes #92 check if array before using array_merge

Also updated the doc bloc to the $bodyParams param, and set a default value of null to better represent what may get passed in.
Bug fix incompatible declaration of request method

`$body` is now null, not an array.
This definition caused an error.

Declaration of OAuth\OAuth2\Service\Foursquare::request() must be compatible with that of OAuth\Common\Service\ServiceInterface::request()
Merge pull request #94 from thomaswelton/patch-2

Bug fix incompatible declaration of request method
Merge pull request #93 from thomaswelton/patch-1

Fixes #92 check if array before using array_merge
Don't prematurely start Symfony session
Fix Foursquare bug
Merge pull request #97 from jenssegers/master

Foursquare service bug
Merge pull request #95 from gigablah/symfony-session-fix

Don't prematurely start Symfony session