Fix handling of schema extensions when filtering coerced values in SCIMMY.Types.SchemaDefinition

[sleelin]

Currently, filtering of included or excluded attributes by the SchemaDefinition type class before return does not correctly handle namespaced extension attributes. This is due to the double handling of extension schema filtering, and insufficient evaluation of overall filter structure. A specific example of this is evident when making a request for a User resource, extended with the EnterpriseUser schema, and setting excludedAttributes to any attribute from the User schema.

The internal filter method of the SchemaDefinition class has been rewritten to first capture all excluded and included attribute filters. If no explicit inclusions were specified, and all supplied filters are exclusions, all attributes and extensions not explicitly excluded will instead be included (fixes #37). Double handling of extension schema filtering has also been resolved, as inclusions and exclusions will only be evaluated when the filter condition is not for a namespaced attribute. Test fixtures for the coerce method of the SchemaDefinition class have also been updated to verify attribute, extension, and namespaced attribute filtering is behaving as expected.