getResponse() throws warning if RESULTTEXT is not present

[martinbohmcz]

After fixing #11 another problem has arised.

If RESULTTEXT is not present at all in the response, the provide() call triggers the following warning:

Undefined array key "RESULTTEXT"

Location: https://github.com/Pixidos/gpwebpay-core/blob/89bc3e513ff28a0e113778822a010d85af86ed53/src/Data/Response.php#L166

[martinbohmcz]

Today it has happend again. The response came with PRCODE 30 (Declined in AC) and SRCODE 9500 (Platbu nelze nyní schválit, ale je možné později zkusit platbu znovu). In that case the RESULTTEXT is not present and warning mentioned above is triggered.