Fix wrong descriptions in the unordered collection matcher.

Matching the collection always worked on the same mutable array instance of
matchers. The second round of matching (for the description) therefore
reported the wrong mismatch.

The tests did not reveal this problem, because the assert method exercised
the matching only once.
Merge pull request #29 from nschum/unorderedmismatch

Fix wrong descriptions in the unordered collection matcher.