Allow single IPTC keyword in metadata

[liam-lloyd]

Currently, we expect the IPTC:Keyword field in metadata files to be an object containing potentially multiple keywords. However, in some files it is simply a single keyword expressed as a string. This commit updates the metadata attacher lambda to handle that case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (cb47369) to head (f86140a).

Additional details and impacted files

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##             main     #778   +/-   ##
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  Coverage   98.56%   98.56%           
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  Files         109      109           
  Lines        2778     2778           
  Branches      463      464    +1     
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  Hits         2738     2738           
  Misses         36       36           
  Partials        4        4           

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[cecilia-donnelly]

Is there a reason to have such a long XML file as the fixture? (It seems like a simpler one would be less likely to break for some other reason unrelated to this specific test)

[liam-lloyd]

Is there a reason to have such a long XML file as the fixture? (It seems like a simpler one would be less likely to break for some other reason unrelated to this specific test)

Most of the fixture metadata files used in this package are derived from a real one (from a test environment) in pursuit of authenticity, but it does make sense to a use a smaller one for specific test case like this.