Hardening/136

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/136

Additionally, minor modifications have been done in the main README regarding missing Flume event headers.
Not sure what you mean here. Is this it?:

The <b>content-type</b> header is a replica of the HTTP header. It is needed for the different sinks to know how to parse the event body. In this case it is JSON.