FCP XML: Da Vinci resolve can't load files written by adapter

Before or after PR #494, FCP XML written by otio can have issues loading in Da Vinci Resolve.

To Reproduce:
1. `otioconvert -i tests/sample_data/premiere_example.xml -o ~/premiere_example.otio.xml`
1. Select File->Import Timeline->Import AAF, EDL, XML...
1. select `premiere_example.otio.xml`

Loading the original `premiere_example.xml` works, the passthrough one does not.

Reproduce the failure with tests/sample_data/premiere_example.xml and the otioconvert command described in the issue, then compare that output with the original XML in Da Vinci Resolve. Review the changes from PR #494 and the FCP XML adapter entry point. Done means the passthrough output loads successfully in Resolve.

Written by the indexing model from the issue text.