Add type check when validating definition

[ngaya-ll]

This prevents an AttributeError when validating definitions like this:

type: object
properties: ...
additionalProperties: false

[coveralls]

Coverage increased (+0.006%) to 98.563% when pulling 91af7a8 on ngaya-ll:additional-properties-bool into 7234ed9 on Yelp:master.

[ngaya-ll]

#138 added support for boolean additionalProperties which invalidates one of the test cases in this PR. I'm not sure whether this is correct as the Swagger 2.0 spec is a bit vague about that, see OAI/OpenAPI-Specification#668.

[macisamuele]

I'll try to have a look to the PR and the Specifications later this evening.

For what I remember the schemas defined in #/definitions/<key> must be dictionaries, and the PR seems ensuring this but I'm not 100% sure. So I need to validate this and check the code change.

@ngaya-ll thanks for the PR and for holding tight with us until a proper review is possible.

[sjaensch]

I remember us looking into additionalProperties and the fact that it can be a boolean value in the past, it just slipped through during the code review for #134. OAI/OpenAPI-Specification#668 mentions that (more explicit) support for a boolean value was added in OpenAPI 3.0. See OAI/OpenAPI-Specification#894.

Given all of that, we should allow boolean values for additionalProperties in OpenAPI 2.0.