Online membership signup with free membership fails

• • • • This is a regression from 4.4 ***

To recreate:

• Add a $0 fee membership type
• Update sample contribution page id=2 to include that membership type in the membership block
• Use the online contribution page to signup for the free membership

The transaction appears to complete, but no membership record and no line_item record is created.

Same test in 4.4 results in a membership record (but no line_item record). Equivalent flow for front-end event registration in 4.4 and 4.5 creates a participant record, a $0 contribution record and a $0 line_item record. I think this is what we should do in 4.5 for $0 membership. (But minimally the membership record should definitely be created to match 4.4 behavior.)