Make get_object_terms work with and without object caching.

[joshlevinson]

When get_object_terms resorts to using get_the_terms, get_object_term_cache is called.
The data stored by update_object_term_cache is an associative array, the key being the term id.
This causes $name[0]->slug to attempt to access an array item that doesn't exist.
This PR makes use of the key function to get the first item from the array regardless of whether it's associative or not.

[joshlevinson]

I did some poking around because this behavior felt unnatural to me—that get_the_terms can return two different types of results depending on whether or not the results are present in the object cache. It's definitely a requirement to keep the associative array style based on how wp_cache_add works ($key, $data, $group - $key being the term_id).

However, I feel like the behavior should still be consistent. This leaves the only option being to change wp_get_object_terms so it returns an associative array instead of the current indexed one.

I compared the taxonomy functions with how metadata is retrieved to determine whether they currently behave similarly (in which case I would think wp_get_object_terms should be left alone).
get_metadata pulls from either wp_cache_get or update_meta_cache, which will both yield an associative array with the meta_id being the key. Taxonomies are different in that they're arrays of objects, so they have identifiers without using an array key. I still feel that taxonomies could benefit from a similar pattern though.

Does this sound like a good first trac ticket for me to take on?

[jtsternberg]

Josh, hmm these are good thoughts, and that does sound like a good trac ticket to open, though one may exist already. Let me know what you find!

[jtsternberg]

And thanks for the PR!