Add HTTP PATCH method to the REST APIs

[sebymiano]

In the current implementation, the HTTP PUT method is used also to update the value of an element and/or collection. This is a wrong behavior because according to this url the PUT method is used to replace the element or collection, or if it does not exist, create it.

We should instead add the PATCH method, which is used to modify an existing HTTP resource.

[frisso]

@sebymiano : please sync with Ivano, because it seems to me that "PATCH" requires a sort of "diff" in the body (i.e., what to remove, what to add). Not 100% sure, but please check with him.