fixed issue with pointer to scalar values

[jhaynie]

If you have a response struct like the following:

struct Foo {
   Bar *string  `json:"bar"`
}

And Bar is nil. The result will be incorrectly encoded on returning. The result will be \u003cnil\u003e instead of null (when serialized to JSON).

This fix will fix isNullish to check for *string type and other primitive types.

[coveralls]

Coverage decreased (-0.03%) to 82.573% when pulling 2dd87f4 on jhaynie:master into 44a52a8 on graphql-go:master.

[coveralls]

Coverage decreased (-0.1%) to 82.478% when pulling 7b41a76 on jhaynie:master into 44a52a8 on graphql-go:master.

[coveralls]

Coverage decreased (-0.6%) to 82.046% when pulling 1fc7c7c on jhaynie:master into 44a52a8 on graphql-go:master.

[chris-ramon]

LGTM 👍 — thanks a lot @jhaynie