[PERSISTENCE] Forward document type to mappings

[miguelff]

This PR tries to fix the bug described in #270

The problem was the following.

• Once a model extends Persistence::Model it sets up two attributes. As a consequence, the attribute macro instantiates a Elasticsearch::Model::Indexing::Mappings in the model. This instance contains an attribute @type that defaults to the model name in lowercase.
• Any other definition of document_type will override the document_type in the gateway delegate, but the mappings instance originally created in the previous step remains untouched, and hence in an inconsistent state.

What I did here is: Instead of simply forward document_type to the gateway, also change the mappings' document_type accordingly, leaving it in a correct state.

An integration test was modified, in order to have a regression of the fix.

[karmi]

Waiting for unit tests, otherwise good to merge!

[miguelff]

@karmi Done.

[karmi]

Thanks, Miguel!, merged.