Adds tests for attributes.is_attr_set

Adds tests for attributes.is_attr_set

ATTR_NOT_SPECIFIED and None are singleton objects

When conducting "is in" against a list of objects, they are compared using
unequal unless they are the same object.
http://docs.python.org/2/reference/expressions.html#in

According to PEP8, they should be compared by their id not value
Therefore the is_attr_set logic has changed to:
    return not (attribute is None or attribute is ATTR_NOT_SPECIFIED)

Wrote unittests for is_attr_set

Change-Id: I2ebdd29564e5627b9e33b13a744948e8c9c0b9ce