`taxonomy` param of `WP_Term_Query` doesn't accept string value

[boonebgorges]

...even though it says it does.

See ​https://wordpress.org/support/topic/passing-a-single-taxonomy-as-a-string-to-wp-term-query-returns-error

I vaguely remember it being a conscious choice not to accept string values for taxonomy, but somehow it snuck into the documentation. In retrospect, I think it's fine (and consistent with other Query classes) to accept a string here.

[boonebgorges]

37545.diff​ contains tests for array vs string values, as well as a simple fix.

Ping @ocean90 for review.

[ocean90]

37545.diff​ looks good to me. Core does the same in get_terms(). (The extra check for null is ​superfluous.)

[boonebgorges]

In 38181:

In WP_Term_Query, accept a string value for taxonomy.

Props endocreative.
Props ocean90 for review.
Fixes #37545.

[boonebgorges]

In 38182:

In WP_Term_Query, accept a string value for taxonomy.

Merges [38181] to the 4.6 branch.

Props endocreative.
Props ocean90 for review.
Fixes #37545.