Bumped version v1.3.16
Make Listing#respond_to? match #method_missing for Standard Fields

The Listing model uses `method_missing` to allow shortcut access to
standard fields. This change extends that functionality so that
`responds_to?` correctly reports on those shortcut methods.
Merge pull request #79 from mjobrien/listing_sf_respond_to

Make Listing#respond_to? match #method_missing for Standard Fields
RUBYAPI-34 Made ListingCart detect dirty attributes and fixed the
tests