Restore the ability to create arbirtary meta tags.

This behavior was lacking a test case, and was accidentally removed in
a5feef9.

Refs #4903
Allow arbitrary meta tags to be created with arrays.

This was removed in the 2.x -> 3.0 updates, but there isn't really
a reason to remove it imo.

Refs #4903
Merge pull request #4906 from cakephp/issue-4903

Restore the ability to create arbirtary meta tags.