fix non-symbol keys hash. (fixed at r49088, 49089)
ruby 2.2.0dev (2014-12-26 trunk 49030) [x86_64-darwin13]
The issue was already fixed on trunk at r49088.
This ticket is only for backport management.
I've confirmed that this isn't reproduced on 2.1.5 and 2.0.0-head.