Feature/iotcore 110 restrict proxy function scope

Address the following issues:

https://github.com/telefonicaid/fiware-orion-pep/issues/7
https://github.com/telefonicaid/fiware-orion-pep/issues/6
https://github.com/telefonicaid/fiware-orion-pep/issues/3


I usually prefer to use camel case in header names, as I find it more readable, but Express makes all the names of the headers lowercase upon arrival, to make it easier to check for their names. If you find it more consistent to use lower case in both places it's ok for me.