Override bp_options.php more consistently

In cases where this is used without putting any settings in bp_options.php, the library allows overriding any of the options in bpCreateInvoice() except for the verify hash, which still reads from just the pre-merged options array.

Even though you can override options in bpCreateInvoice, you can't override anything in the verification function, so I added the same array merge that was in the invoice creation to the verification, then modified the variables to used the overridden, merged array.