hash.c: refactor env_enc_str_new
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59449 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
hash.c: refactor env_enc_str_new
without intermediate string, and set econv flags if default
internal encoding is set too.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59449 b2dd03c8-39d4-4d8f-98ff-823fe69b080e