where the code reads better with is_multiple

Two error conditions:
Multiple terms passed from different taxonomies
Multiple terms passed when taxonomy does not support multiple terms

When a term is related to two items and is removed from one of them,
the other item should not show as being modified.

This one covers the case where an item is removed from a term.

Also added test intended to illustrate why I included the test for
is_optional

http://doughellmann.com/2012/11/12/the-performance-impact-of-using-dict-instead-of-in-cpython-2-7-2.html