fix behaviour for TimeWithZone#utc at end of DST

- The #utc method should not raise an ambiguous time exception when converting
  a Teasy::TimeWithZone from local to utc since it knows the time zone and
  period.
- Add regression test
- Add test to assert that #in_time_zone at the start of DST returns a timestamp
  with DST period

lib/teasy/time_with_zone.rb

@@ -35,6 +35,18 @@ def self.from_utc(utc_time, zone = Teasy.default_zone)
       from_time(utc_time, 'UTC').in_time_zone!(zone)
     end
 
+    def self.parse(string, zone = Teasy.default_zone)
+      from_utc(Time.parse(string).utc, zone)
+    end
+
+    def self.iso8601(string, zone = Teasy.default_zone)
+      from_utc(Time.iso8601(string).utc, zone)
+    end
+
+    def self.strptime(string, format, zone = Teasy.default_zone)
+      new(*DateTime._strptime(string, format).values, 'UTC').in_time_zone!(zone)
+    end
+
     def in_time_zone!(zone = Teasy.default_zone)
       time = utc_time
       @zone = TZInfo::Timezone.get(zone)
@@ -57,7 +69,7 @@ def utc?
     end
 
     def utc!
-      @time = @zone.local_to_utc(@time)
+      @time = @zone.local_to_utc(@time, @period.dst?)
       @zone = TZInfo::Timezone.get('UTC')
       @period = @zone.period_for_local(@time)
       self

test/teasy/time_with_zone_test.rb

@@ -69,6 +69,13 @@ def test_in_time_zone
     refute_equal time.object_id, @timestamptz.in_time_zone('Asia/Calcutta')
   end
 
+  def test_in_time_zone_at_dst_start
+    dst_start = Teasy::TimeWithZone.new(2014, 3, 30, 1)
+      .in_time_zone!('Europe/Berlin')
+    assert_equal 3, dst_start.hour
+    assert dst_start.dst?
+  end
+
   def test_raises_on_ambiguous_time
     dst_end = [2014, 10, 26, 2, 0, 0, 0, 'Europe/Berlin']
     assert_raises(TZInfo::AmbiguousTime) do
@@ -382,6 +389,17 @@ def test_usec
     assert_equal 1, @timestamptz_berlin.usec
   end
 
+  def test_utc_at_dst_end
+    first_ambiguous_timestamp = Teasy::TimeWithZone.new(2014, 10, 26)
+      .in_time_zone 'Europe/Berlin'
+    assert_equal 2, first_ambiguous_timestamp.hour
+    assert_equal 0, first_ambiguous_timestamp.utc.hour
+    last_ambiguous_timestamp = Teasy::TimeWithZone.new(2014, 10, 26, 1)
+      .in_time_zone 'Europe/Berlin'
+    assert_equal 2, last_ambiguous_timestamp.hour
+    assert_equal 1, last_ambiguous_timestamp.utc.hour
+  end
+
   def test_utc
     assert_instance_of Teasy::TimeWithZone, @timestamptz.utc
     assert_instance_of Teasy::TimeWithZone, @timestamptz_berlin.utc