fixed a bug in POST /v1/contextEntities

### Description
The type of the entity was not passed to the lower layers and thus not saved in the database.
It was lost.  The fix was more than easy, but as the case was not covered in functests, a new functest had to be added.

Fixes issue #684.

The case described in https://github.com/telefonicaid/fiware-orion/issues/684 includes Fiware-Service and Fiware-ServicePath. I'd suggest to complement this .test with another using also these headers (just I feeling... I haven't look in detail #684 so I'm not sure if what I say makes fully sense).