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Change into_maybe_owned to into_cow

Recent nightlies have deprecated MaybeOwned in favor of CowString.
Merge pull request #65 from Gekkio/maybeowned-to-cow

Change into_maybe_owned to into_cow
0.2.4: language changes.

`MaybeOwned` has been superseded by `CowString`. That really doesn't
affect rust-encoding, as it used `SendStr` which now refers to
a specialization of `CowString`, but it has also removed
`IntoMaybeOwned` so we needed to update.
Merge remote-tracking branch 'upstream/master'