Fix for issue #4582, plus simple test

lib/jekyll/filters.rb

@@ -315,7 +315,7 @@ module Jekyll
     def time(input)
       case input
       when Time
-        input
+        input.clone
       when Date
         input.to_time
       when String

test/test_filters.rb

@@ -103,6 +103,13 @@ class TestFilters < JekyllUnitTest
         should "format a time according to RFC-822" do
           assert_equal "Wed, 27 Mar 2013 11:22:33 +0000", @filter.date_to_rfc822(@sample_time)
         end
+
+        should "not modify a time in-place when using filters" do
+          t = Time.new(2004, 9, 15, 0, 2, 37, "+01:00")
+          assert_equal 3600, t.utc_offset
+          @filter.date_to_string(t)
+          assert_equal 3600, t.utc_offset
+        end
       end
 
       context "with Date object" do