Restrict stripped chars in testXMLAttribute

This was preventing things like an entity-encoded newline from being the value of an attribute. Performing a strip, due to when this operation occurs after it is translated into a python string, some meaning is lost here.

My guess is the original intent was to remove whitespace that typically occurs in XML elements, but in attributes it makes less sense, and in fact what I've changed it to here may not be the final solution because it cannot handle tabs. Maybe there should be no strip at all.

Either way, this was preventing some IOOS/SWE related work I'm cleaning up.