simplify handling of primitives

[bttmly]

Resolves #9

Object(x) !== x is a terse idiom for detecting if something is a primitive value

I removed the handling for schema instanceof String since it seems like that case should either be extended to schema instanceof Boolean and schema instanceof Number or removed entirely, with the second option being simpler. Further, there was not a test on it.

If you want the same behavior for undefined without accepting these changes, the added test will pass if you change === null on L48 to == null.