Should not accept an optional property with value null (unless null type)

[cburgmer]

It is my understanding that null values should be invalid, unless the null type is allowed. This matches (failing) examples like {"type": "string"} with null on http://jsonschemalint.com/draft4/.

However currently

user=> (schema/validate {:type "string"} nil)
true

This commit changes the behaviour to disallow null, removing code previously allowing null of the property was optional (maybe as specified in an older draft?).

I have only added tests for string and integer types so far.

[jaor]

Looks good to me: merged!