Implement Requirement 1.3.4 (type mismatch check in client)

[maxveldink]

While reviewing #124, I realized that we were doing too much work in our InMemoryProvider implementation for verifying type mismatches. In fact, Requirement 1.3.4 indicates that the client should ultimately be checking this for any provider so we don't need to put this guarantee in the individual provider implementations.

We should ensure this requirement is implemented in the client. Then, we should simplify the InMemoryProvider.

[mschoenlaub]

We should ensure this requirement is implemented in the provider.. @maxveldink Shouldn't this say client?

[maxveldink]

Yes, good catch!