Bug/convop metadatas

Fixed a few bugs:
* **GET /ngsi10/contextEntity/entity1/attributes/attrib1/valueID** now works correctly. Previously the broker would only return the first attribute, regardless of which one you specified in the URL.
* **POST /ngsi10/contextEntity/entity1** now works correctly in JSON. There was a misspelling in the parsing vector that prevented the detection of attribute metadata, and it would not get saved into the DB. This affected both POST Individual Context Entity and POST Individual Context Entity Attributes.
* **PUT /ngsi10/contextEntity/entity1/attributes/attrib1/valueID** now works. It wasn't able to read the value specified in the body of the request. Now it does.

Note that there's still a few StatusCode response messages that are not rendered correctly (PUT & DELETE). This can be solved when https://github.com/telefonicaid/fiware-orion/pull/278 is merged.