Don't try to convert unicode types to str types.

I ran into a problem similar to issue #1 and I think this should solve it.  Or at least get a little closer to a solution.  I can't claim to have done exhaustive testing, but my case (database record with a utf8 character) and the test case from #1 both work with these changes.  And the output of `python texttable.py` appears to still be the same after these changes.