Handle string types for InstanceActionEvent exc_tb serialization

Commit 59a6cf233b538d6666740de4796fce25ed8265aa added code to handle
serializing non-str exception traceback objects, but didn't account for
unicode.  This change uses six.string_types to handle str and unicode
objects for the exc_tb argument.

Adds a new unit test for the unicode case and firms up two existing
tests for how traceback.format_tb is mocked (or shouldn't be in the case
of exc_tb being a str).

Closes-Bug: #1327476

Change-Id: Icc10b62a3f65610c50e86c0b366c2b70b1a0932d
Merge "Handle string types for InstanceActionEvent exc_tb serialization"