Metadata stack tests

Hey,

Here's a test for the Context metadata stack.

I changed when the classMetadata is pushed to the stack so that it's consistent with when the propertyMetadata is pushed. (Now they are both pushed after the exclusion strategy has been called. Before, the class metadata was added before the exclusion strategy was actually called.)

Please look at the 2nd commit diff to have something readable (since the first is a rename + cs) : https://github.com/adrienbrault/serializer/commit/f226163cbebb1ff475063bc02aa437efc93230c4