send_file now takes :last_modified option

fixes #131

lib/sinatra/base.rb

@@ -164,7 +164,7 @@ module Sinatra
     # Use the contents of the file at +path+ as the response body.
     def send_file(path, opts={})
       stat = File.stat(path)
-      last_modified stat.mtime
+      last_modified(opts[:last_modified] || stat.mtime)
 
       if opts[:type] or not response['Content-Type']
         content_type opts[:type] || File.extname(path), :default => 'application/octet-stream'

test/helpers_test.rb

@@ -419,6 +419,13 @@ class HelpersTest < Test::Unit::TestCase
       assert_equal File.mtime(@file).httpdate, response['Last-Modified']
     end
 
+    it 'allows passing in a differen Last-Modified response header with :last_modified' do
+      time = Time.now
+      send_file_app :last_modified => time
+      get '/file.txt'
+      assert_equal time.httpdate, response['Last-Modified']
+    end
+
     it "returns a 404 when not found" do
       mock_app {
         get '/' do