WP_Term_Query breaks when cache is returned

[wonderboymusic]

Found this bug on NYT in production since I am running the RC there #yolo

as originally reported here:
​https://wordpress.slack.com/archives/core/p1470433813002336

$cache = wp_cache_get( $cache_key, 'terms' );
if ( false !== $cache ) {
    if ( 'all' === $_fields ) {
        $cache = array_map( 'get_term', $cache );
    }
    return $cache;
}

Needs to be:

$cache = wp_cache_get( $cache_key, 'terms' );
if ( false !== $cache ) {
    if ( 'all' === $_fields ) {
        $cache = array_map( 'get_term', $cache );
    }
    $this->terms = $cache;
    return $cache;
}

[wonderboymusic]

In 38211:

Taxononmy: add unit that shows that WP_Term_Query is broken when the query is read from the cache.

See #37591.

[wonderboymusic]

In 38212:

Taxononmy: set WP_Term_Query::terms when returning terms from the cache in WP_Term_Query::get_terms().

See #37591.

[boonebgorges]

[38211] and [38212] look good to merge to the 4.6 branch.

[ocean90]

In 38214:

Taxononmy: Set WP_Term_Query::terms when returning terms from the cache in WP_Term_Query::get_terms().

Merge of [38211] and [38212] to the 4.6 branch.

Props wonderboymusic.
Props boonebgorges for review.
Fixes #37591.