Return the correct seconds since the epoch value for strftime with %s.

%s was being deferred to the result of the conversion to local. This is
actually a UTC instance being used to represent local time, so the
result would be the number of seconds since the epoch plus the total UTC
offset.

Override the handling of %s and use the passed in UTC instance to
calculate the value.

Resolves #91.

lib/tzinfo/timezone.rb

@@ -573,15 +573,19 @@ def current_period_and_time
     # depending on the version of Ruby in use (for example, %:::z is only
     # supported by Time#strftime from MRI version 2.0.0 onwards.)
     def strftime(format, utc = Time.now.utc)      
+      utc = TimeOrDateTime.wrap(utc)
       period = period_for_utc(utc)
-      local = period.to_local(utc)      
-      local = Time.at(local).utc unless local.kind_of?(Time) || local.kind_of?(DateTime)
+      local_wrapped = period.to_local(utc)
+      local = local_wrapped.to_orig
+      local = local_wrapped.to_time unless local.kind_of?(Time) || local.kind_of?(DateTime)
       abbreviation = period.abbreviation.to_s.gsub(/%/, '%%')
 
-      format = format.gsub(/%(%*)(Z|:*z)/) do
+      format = format.gsub(/%(%*)([sZ]|:*z)/) do
         if $1.length.odd?
           # Escaped literal percent or series of percents. Pass on to strftime.          
           "#$1%#$2"
+        elsif $2 == "s"
+          "#$1#{utc.to_i}"
         elsif $2 == "Z"
           "#$1#{abbreviation}"
         else

test/tc_timezone.rb

@@ -1260,6 +1260,7 @@ def test_strftime_datetime
     assert_equal('BST BST', tz.strftime('%Z %Z', dt))
     assert_equal('BST %Z %BST %%Z %%BST', tz.strftime('%Z %%Z %%%Z %%%%Z %%%%%Z', dt))
     assert_equal('+0100 +01:00 +01:00:00 +01 %::::z', tz.strftime('%z %:z %::z %:::z %::::z', dt))
+    assert_equal('1153001522 %s %1153001522', tz.strftime('%s %%s %%%s', dt))
   end
 
   def test_strftime_time
@@ -1271,6 +1272,7 @@ def test_strftime_time
     assert_equal('BST BST', tz.strftime('%Z %Z', t))
     assert_equal('BST %Z %BST %%Z %%BST', tz.strftime('%Z %%Z %%%Z %%%%Z %%%%%Z', t))
     assert_equal('+0100 +01:00 +01:00:00 +01 %::::z', tz.strftime('%z %:z %::z %:::z %::::z', t))
+    assert_equal('1153001522 %s %1153001522', tz.strftime('%s %%s %%%s', t))
   end
 
   def test_strftime_int
@@ -1282,6 +1284,7 @@ def test_strftime_int
     assert_equal('BST BST', tz.strftime('%Z %Z', i))
     assert_equal('BST %Z %BST %%Z %%BST', tz.strftime('%Z %%Z %%%Z %%%%Z %%%%%Z', i))
     assert_equal('+0100 +01:00 +01:00:00 +01 %::::z', tz.strftime('%z %:z %::z %:::z %::::z', i))
+    assert_equal('1153001522 %s %1153001522', tz.strftime('%s %%s %%%s', i))
   end
 
   def test_get_missing_data_source