validateResponse does not account for specification extensions that are null

[todanator]

#74

A responses object with a Specification Extension with the type null will throw and error

Cannot read property 'headers' of null

You can see here that responses should be extendable
https://swagger.io/specification/#responsesObject

h2. Test
Example responses object

"responses": {
	"x-an-extension-null": null,
	"200": {}
}

'response' will evaluate to null here:
/~https://github.com/BigstickCarpet/swagger-parser/blob/master/lib/validate-spec.js#L49
And throw the error here for trying to access null.headers
/~https://github.com/BigstickCarpet/swagger-parser/blob/master/lib/validate-spec.js#L251

Suggest either filtering extensions or null safe access of headers.

[JamesMessinger]

Thanks for reporting this issue and submitting a PR. I've merged your changes, and they should be published to npm within the next few days