Avoid defining multibyte method names in JSON decoding test for JRuby Compatitibility

This change is similar to #11736 & in same way switched with fixed string & the index of the hash for method name. the index was added because otherwise, ruby will raise Error.

**Before this change**:
>JRuby: raised an `NoMethodError` when running the decoding tests.
>MRI: all green.

**After this change**
>JRuby: All green :+1:  
>MRI: all green(no change)