Add back select argument to DynamicURLStore.get_metadata

It seems to have been accidentally removed in c41675d.
Call store.get_metadata with `select` in JoinedStore.get_metadata
Fix bug in DictMemoryStore that was breaking JoinedStore tests.
Merge pull request #1 from enthought/bugfix/joined_set

Fix bug in DictMemoryStore that was breaking JoinedStore tests.
Merge pull request #58 from punchagan/fix-broken-get-metadata

Add back select argument to DynamicURLStore.get_metadata