Issue #4985 - fix NPE related to use of Attributes.Wrapper getAttributeNameSet()

[lachlan-roberts]

Issue #4985

This PR tries to prevent any of the Attributes.Wrapper implementations from returning a value in getAttributeNameSet() that corresponds to a null attribute when getAttribute() is called with the name.

For each Attributes.Wrapper we now do the following:

• Ensure that if the value is set to null, we do not add the corresponding attribute name to the set.
• Don't determine attribute values lazily as we won't know whether its null or not.
• Ensure that any conflicting attributes from the wrapped Attributes are removed.

@mpetzold has tested this branch and it seems to have fixed the NPE originally reported
see #4985 (comment)

The NPE is gone. However, I still have a problem with the WebSocket connection of Vaadin / Atmosphere. This may be something else or related configuration.

[gregw]

LGTM but you can remove the TODO before merging