* time.c (time_set_utc_offset): reset zone when setting to fixoff
  mode.  while previously TZMODE_SET_FIXOFF has reset it always,
  the zone is kept for loaded zone since r65025.
  [ruby-core:90627] [Bug #15439]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66520 b2dd03c8-39d4-4d8f-98ff-823fe69b080e