Test rejection of bad JSON.

(1) Added a few test cases to see if checks in [`Decode()` and `DecodeInto()`](https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/api/helper.go) were working as indicated.

(2) Did not include the case of invalid JSON syntax, such as the use of a number as a key. I was not sure if such a case is outside the scope of these tests.

(3) I kept with the terminology of "known type" in reference to the terminology used for the `map` in `helper.go`.