Fix whitespace
Explicitly use http_build_query on post fields

This could fix the issue of:

I found the issue and it seems to be on your end, somehow you’re
merging the cardnumber, expdate and callback_url parameters in one.

This is your request logged at our end:
client_id=635092&api_key=********************&test_transaction=1&cardnum
ber=411111**************************************************************
****************12311&browser_useragent=Mozilla/5.0 (Macintosh; Intel
Mac OS X 10_9_2) AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/33.0.1750.152
Safari/537.36&refernece=GBV57U&amount=40.91&currency=GBP