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

New pull request based on latest version of QIIME codebase.
This seems unlikely to happen in practice, but it might be safer to check whether `mothur_taxon` is in `self._original_taxa` already, in case replacing spaces with underscores creates two original taxa that map to the same mothur-compatible taxon.

An extremely contrived example: `foo<space><space>;<space>bar` and `foo_<space>;<space>bar` would both be converted to the same thing (`foo__;_bar`), while in the original taxonomy they'd be considered distinct taxa.

Maybe in this case an error should be raised?