Fixes #1025, spaces in id-to-taxonomy file break assignment with Mothur.

New pull request based on latest version of QIIME codebase.
So tough to handle temp files correctly in tests.  This is a bit tricky to handle in tearDown because output_fp might not be present if an error occurs before writing the output.  I'd need to manage it separately in tearDown and call remove_files(..., error_on_missing=False).

Would rather wrap call to assigner in try/finally block -- will do this in next commit.  Happy to follow other examples here if others use a different strategy.