Add possibility to remove headers

[asimell]

Currently Set Headers only updates the headers dictionary. There is no option to remove a header once it's set. Either Set Headers should support unsetting a header or there should be a new keyword that removes a given header from headers.

Functionality could be e.g.

# Assume headers {"a": "1", "b": "2"} are set
Remove Header    a    # removes header "'a': '1'"
Remove Header    c    # does nothing as there's no such header set OR raises an error
Remove Header    c    strict=${FALSE}    # Same as above, but doesn't raise an error