test_time.rb: encoding of Time#zone

* test/ruby/test_time.rb (assert_zone_encoding): encoding of
  Time#zone is not locale, but the default internal.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46906 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
time.c: unnecessary encoding

* time.c (time_zone_name): remove unnecessary encoding and
  conversion if it is 7bit-ascii only.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46907 b2dd03c8-39d4-4d8f-98ff-823fe69b080e