Fixed issue #30. Now migrating to Dexie is easy.

Now the unit test for issue #30 pass.
(an error on one assertion in the unit test was also corrected)

This fix makes Dexie adjust to existing databases created by other
libraries. Before there was a requirement that the names of the indexes
had to match their corresponding keypaths. Now Dexie will adapt the
existing index names while still keeping its simple
schema API and only specifying keyPaths like before. The real name of
the index is only adapted internally when doing the querys.