support named interface to Field.CallArgs

[vvakame]

types.IsInterface returns true when passed value's underlying type is interface.
its too much in this situation.
so, if argument type is something like type ID interface { isID() }, types.IsInterface returns true. it will generate unexpected code.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage decreased (-0.08%) to 74.997% when pulling dc6c304 on fix-named-interface into 3049369 on master.