There is a cyclical reference here where the matcher holds a list of
requests in history and the requests hold a reference to the matcher.
Break this using a weakref so that it can be cleaned up easily.
In practice I don't see this being a problem as people will want to test
that value as part of the test where the mock is still active.
There has not been a release with this functionality so there is no
compatibility change.
Change-Id: Id1669ea40a48d09432367646a0004866947ff72b
ace78cd9c9