PATCH routes: support field_attributes

[mattwiebe]

The fields_attributes in api/v1/update_credentials are submitted in a multipart/form-data hash like:

# Simplifying to not show the boundary
fields_attributes[0][name]
Some Title
fields_attributes[0][value]
Some Value
fields_attributes[1][name]
Another Title
# etc

We were only handling the simple value type before.

This is required for Extra Fields editing in ActivityPub: Automattic/wordpress-activitypub#788

[akirk]

I guess the ([^\[]+) is technically correct but could it be more readable to use something like ([a-zA-Z0-9_]+) for easier readability?

[akirk]

Or even (\w+) as per the docs:

\w
any "word" character
A "word" character is any letter or digit or the underscore character

[mattwiebe]

I agree that it's pretty ugly! I wasn't sure how permissive to be on this. I'm also going to reduce the surface area of what this code handles: only enough to support our use-case.

I'm slowly working up a patch for Core WP that'll patch things at the REST API level and then hopefully we can strip this all out.

[mattwiebe]

@akirk finally had a chance to circle back to this, thanks for your feedback, I hope it's good to go!