Allow contrib modules to alter data in PaymentOffsiteForm before API call

This patch is against the 2.1.x branch as I had issues running the dev branch.

Hi, thanks for raising this. I was considering adding an event there as it happens, so it is timely. However there may be a simpler way to fix this particular issue. Please can you tell me what the output of the following is if you add it to the PaymentOffsiteForm:

dpm($this->currentRequest->getBasePath())
dpm($this->currentRequest->getPathInfo());
dpm($this->currentRequest->getRequestUri());
return $form;

(I would check myself except it involves going through the whole process of setting up a new site, so it is probably quicker if you tell me.)

Thanks