Fix wp_post_meta content

[nickmoreton]

Fix wp_post_meta content

The last merge has caused an issue that wp_post_meta no longer contains required fields for import hooks.

I'm adding test to highlight the issue first then add in a fix for the clean_wp_post_meta() method.

The problem was that we altered clean_wp_post_meta() to only save wp:postmeta data. We needed to also be saving all other data for an XML item tag. With the exception of content:encoded.

I've altered the clean_wp_postmeta() to also include some improvements to dealing with removing leading `and replacing:` charachters.

Ticket URL:

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• Testing
  • [x] CI passes
  • [x] If necessary, tests are added for new or fixed behaviour
  • [x] These changes do not reduce test coverage
• Documentation.
  • [x] This PR adds or updates documentation

[nimasmi]

Thanks for making the code changes, @nickmoreton. I'll leave it up to you for how or whether to resolve the docs example issue. I now approve this PR.