added mtime for Deflater.gzip and fixed gzip spec

lib/rack/deflater.rb

@@ -1,5 +1,6 @@
 require "zlib"
 require "stringio"
+require "time"
 
 module Rack
 
@@ -17,7 +18,8 @@ def call(env)
 
     case encoding
     when "gzip"
-      [status, headers.merge("Content-Encoding" => "gzip"), self.class.gzip(body)]
+      mtime = headers["Last-Modified"] || Time.now
+      [status, headers.merge("Content-Encoding" => "gzip"), self.class.gzip(body, mtime)]
     when "deflate"
       [status, headers.merge("Content-Encoding" => "deflate"), self.class.deflate(body)]
     when "identity"
@@ -28,9 +30,10 @@ def call(env)
     end
   end
 
-  def self.gzip(body)
+  def self.gzip(body, mtime=Time.now)
     io = StringIO.new
     gzip = Zlib::GzipWriter.new(io)
+    gzip.mtime = mtime
 
     # TODO: Add streaming
     # TODO: Consider all part types

test/spec_rack_deflater.rb

@@ -41,7 +41,11 @@ class << body; def each; yield("foo"); yield("bar"); end; end
 
     response[0].should.equal(200)
     response[1].should.equal({ "Content-Encoding" => "gzip" })
-    # response[2].to_s.should.equal("\037\213\b\000J\340mH\000\003K\313\317OJ,\002\000\225\037\366\236\006\000\000\000")
+
+    io = StringIO.new(response[2].to_s)
+    gz = Zlib::GzipReader.new(io)
+    gz.read.should.equal("foobar")
+    gz.close
   end
 
   specify "should be able to fallback to no deflation" do