Allow overriding security spec with empty list

[jlumpe]

Currently the global security property of the spec can be overridden in specific operations, but only if the list of security requirements is non-empty.

In the official spec it states that

To remove a top-level security declaration, an empty array can be used

for an operation's security property.

This change should bring things in line with the official spec.

[coveralls]

Coverage remained the same at 97.34% when pulling d8f7684 on jlumpe:master into 14de09f on Yelp:master.

[macisamuele]

@jlumpe thanks a lot for your contribution and for having identified the specs un-compliant behaviour.

Your change looks correct to me but I would ask if you can add a couple of tests in security_object_test.py that validates your change.

[coveralls]

Coverage remained the same at 97.34% when pulling 5a10c81 on jlumpe:master into 20e685d on Yelp:master.

[coveralls]

Coverage remained the same at 97.34% when pulling b09ce6a on jlumpe:master into 20e685d on Yelp:master.

[jlumpe]

Updated with new test. Test passes this branch, fails on master. Hope I got the code style right.

[macisamuele]

@jlumpe: thanks for the tests ... with them we could be sure that the code change is doing what is expected.