Allow string hash values on AR order method

This behavior has almost no performance impact:

String not allowed  66.910000   0.030000  66.940000 ( 67.024976)
String allowed      69.360000   0.030000  69.390000 ( 69.503096)

Benchmarked with http://git.io/Y0YuRw.
Merge pull request #14263 from robin850/allow_passing_string_to_order_hash

Follow up of #10732 - Allow string hash values on AR order method