add scenario for issue #29
fix for issue #29
Fix import path
Expand issue #29 scenario to cover return values

Since the parameters weren't working properly, the return values
probably don't either - so expand the scenario to show that too.
Fix exprString for interface methods

We weren't printing names for parameters or returns when they were
specified - which doesn't really matter, except that you can have
multiple names per type when it does matter.  It's also neater to
copy the names used by the original code ...

This helps with issue #29.
Fix interface mock generation

Having got exprString right, we now need to fix interface mock
generation when there are multiple return values using names and the
same type.

Kinda issue #29.