Fix a copy/pasted test mistake

Fix a copy/pasted test mistake

Fix a minor test mistake that was copied around a few times.  In mock
setup, the intention was to only return a value of the rpc method used
was a 'call', not 'cast'.  Unfortunately, the variable checked was
method instead of rpc_method, so this expected return was always set
to None.

This also points out that what's being tested here isn't what *really*
was intended.  It ends up testing that the code returns what we mocked
it to return, which isn't all that useful.  However, at least with
this fix it's a bit more clear what it's trying to do.

Change-Id: I18e85aa66f83331318177d70cbae2468ff8bb57d