Fix Network Merchants currency

Use transaction options, not gateway options, for specifying the currency to use

Looks like this has been a bug since this gateway was added :crying_cat_face: 

@ivanfer @girasquid @ntalbott plz 2 review

cc @louiskearns @MonicaGallant @mhashemi86 
This was using the `options` accessor on the gateway itself, which is just the credentials