time.c: zone in fixoff mode

* 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

test/ruby/test_time.rb

@@ -556,6 +556,10 @@ class TestTime < Test::Unit::TestCase
 
   def test_zone
     assert_zone_encoding Time.now
+    t = Time.now.utc
+    assert_equal("UTC", t.zone)
+    assert_nil(t.getlocal(0).zone)
+    assert_nil(t.getlocal("+02:00").zone)
   end
 
   def test_plus_minus_succ

time.c

@@ -1896,6 +1896,7 @@ time_set_utc_offset(VALUE time, VALUE off)
     GetTimeval(time, tobj);
 
     tobj->tm_got = 0;
+    tobj->vtm.zone = Qnil;
     TZMODE_SET_FIXOFF(tobj, off);
 
     return time;