From Ruby ( 2.3.0dev trunk 52520), `Hash#to_proc` is defined
(https://github.com/ruby/ruby/commit/fbe967ec02cb65a7efa3fb8f3d747cf6f620dde1),
and many tests have been failed with
`ArgumentError: wrong number of arguments (given 0, expected 1)`.
Because we call `Hash#to_proc` with no args in `#merge!`.

This commit changes order of conditionals to not call `Hash#to_proc`.