Align 4.6 behaviour with 4.7 for Payment processor buildForm method

In 4.7 the recommended method for payment processors to add js & other resources to a page is by implementing the buildForm hook. This method is also implemented in 4.6 but there is a minor difference. In 4.7 further form building will not occur if the method is implemented AND returns TRUE. In 4.6 it will not occur if the method is implemented.

I actually haven't found a case where you would want to return TRUE - but perhaps iFrames would make sense for that.

In any case, extension writers need to be able to add this in 4.6 in the same way they will in future versions to enable consistent development