Fixed #355 handle null pointer lists

[koblas]

This fixed the previously reported bug dealing with optional lists.

Note: that it changes the unit test for union handling to resolve the now failing union test. @sfriedel, I couldn't find a good reference for the right answer, though since the test returns nil, it shouldn't necessarily be typed (IHMO), I couldn't find a good reference.

	union_interface_test.go:644: Unexpected result, Diff: [
		got: &{map[query:map[pet:<nil> named:<nil>]] []}
		want: &{map[query:map[pet:map[__typename:Cat] named:map[name:<nil> __typename:Cat]]] []}
		]

[coveralls]

Coverage decreased (-0.1%) to 91.798% when pulling 6a8046d on koblas:master into 5c1be08 on graphql-go:master.

[koblas]

@chris-ramon Any chance of looking at this as well?

[chris-ramon]

LGTM 👍 — thanks a lot @koblas 🚢

In regards the union test, we are fine updating it and to not return __typename for that particular use.