Converter::Rename only renames the first element

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/218)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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During review of [pull request #551](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/551), which fixed the `Converter::Remove` function to remove repeated elements, @azeey pointed out that `Converter::Rename` has the same problem in that it only renames the first element and not any repeated elements. I have confirmed this by adding a failing test to the following branch:

* https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/branch/converter_fix_rename?dest=converter_fix_remove#diff

Start with Converter::Rename and the failing test added on the converter_fix_rename branch described in the issue. Confirm the behavior for repeated elements, then verify that the test passes once all repeated elements are renamed.

Written by the indexing model from the issue text.