Fix rating not being parsed

[Maddosaurus]

Hey Evan,

first of all: Thanks for this nice project!

While playing around with it for my use case, I discovered that the Rating (at least in XMP files) is not parsed correctly. It will always stay 0, due to the source var in XMP being a uint8, so the wrong parsing function is used. I've changed the call and can verify that the updated parsing yields the correct results for XMP files generated by Lightroom CC.

Side note: I tried to get the tests that read 1.xmp and its JSON to fail, but was unable to. That might be on my end, so I'd be interested in how the file-based tests work.

If you'd like, I can also provide the XMP to you, just let me know 😄

[Maddosaurus]

@evanoberholster Happy New Year! Do you happen to have some minutes to take a look at this PR?

[Maddosaurus]

@mholt thanks for approving, but it looks like I cannot merge the PR on my own. Would you mind doing the honors? Thanks :heart:

[mholt]

I don't have commit privileges, I just wanted to support your change as I think this project is really useful.