Fixed Client.php for params with special value

Calling request bin was broken with these test values :

array(
  'amount' => 300,
  'capture_day' => 0,
  'capture_mode' => 'PAYMENT_N',
  'data' => 'NB_PAYMENT=3;PERIOD=30;INITIAL_AMOUNT=100'
)

* 'capture_day' was skipped because it equals 0 (fails the "not empty" test, line 76)
* 'data' was truncated because of the semicolon

Now these cases are handled properly.