Only set ModifiedAfter param if present
Only set ModifiedAfter param if present

Fixes bug whereby if gateway.get_invoices is called without a
:modified_since argument, the following URL is called from Xero:

  https://api.xero.com/api.xro/2.0/Invoices?ModifiedSince=

BookingBug/xero_gateway,sharesight/xero_gateway,Flatbook/xero_gateway,amxsv/xero_gateway,armstrjare/xero_gateway,domness/xero_gateway,xero-gateway/xero_gateway,Thinkei/xero_gateway