Use just name instead of firstname & lastname.

**Problem**
We have been getting reports of payflow pro users not receiving customer information in their merchant account.

**Solution**
I believe this was being caused by a change I made to this area a while back (https://github.com/Shopify/active_merchant/pull/599/files). Payflow Link had deprecated their `name` field and was now requiring both a `firstname` and `lastname` separately. When looking at the documentation for payflow pro (https://cms.paypal.com/cms_content/en_US/files/developer/PP_PayflowPro_Guide.pdf), it appears to have the same requirement of separate first and last names, so the same change was made for payflow pro as well.

However, because we actually use their XML api, there is a slightly different documentation (https://cms.paypal.com/cms_content/MX/es_XC/files/developer/PP_PayflowPro_XMLPay_Guide.pdf) which states that both `firstname` and `lastname` should continue to be mapped into a single `name` field for payflow pro.

Review
@odorcicd @jduff 
/cc @BlakeMesdag @louiskearns