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

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.
Merge pull request #11789 from gaurish/multibyte_methods

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