Fix #1366 error when posting with form_params
Fix #1366 error when posting with form_params

HHVM return null when trying to build the query due to null being passed rather than an empty string.
thinsoldier/guzzle,mortenhauberg/guzzle,Tobion/guzzle,BostjanOb/guzzle,kkopachev/guzzle,paveljanda/guzzle,dshafik/guzzle,Brunty/guzzle,michabbb-backup/guzzle,ehough/guzzle,thinsoldier/guzzle,BostjanOb/guzzle,wherefortravel/guzzle,michabbb-backup/guzzle,ehough/guzzle,mortenhauberg/guzzle,guzzle/guzzle,wherefortravel/guzzle,paveljanda/guzzle,guzzle/guzzle,kkopachev/guzzle,Tobion/guzzle,dshafik/guzzle,Brunty/guzzle