Fix factory prop with single key

[thetamind]

Fixes FunctionClauseError when prop used with path with a single key.

Dialyzer correctly emitted a warning:

The call 'Elixir.Kernel':get_in
         (_acc@1 :: any(),
          fun(('get' | 'get_and_update', map(), fun((_) -> any())) ->
                  'pop' | [{atom(), _}] | {_, [{_, _}] | map()} | map())) will never return since the success typing is 
         (any(),
          nonempty_maybe_improper_list()) -> 
          any() and the contract is 
          ('Elixir.Access':t(), [term(), ...]) -> term()

Plus a couple other test fixups.

[trbngr]

Fantastic. I've been bitten by this many times. Could never find the time to get in there!