Index: lib/webrick/httpresponse.rb
===================================================================
--- lib/webrick/httpresponse.rb	(revision 39880)
+++ lib/webrick/httpresponse.rb	(working copy)
@@ -299,9 +299,10 @@ module WEBrick
     # Sends the body on +socket+
 
     def send_body(socket) # :nodoc:
-      case @body
-      when IO then send_body_io(socket)
-      else send_body_string(socket)
+      if @body.respond_to? :readpartial then
+        send_body_io(socket)
+      else
+        send_body_string(socket)
       end
     end
 
Index: test/webrick/test_httpresponse.rb
===================================================================
--- test/webrick/test_httpresponse.rb	(revision 39880)
+++ test/webrick/test_httpresponse.rb	(working copy)
@@ -1,5 +1,6 @@
 require "webrick"
 require "minitest/autorun"
+require "stringio"
 
 module WEBrick
   class TestHTTPResponse < MiniTest::Unit::TestCase
@@ -45,5 +46,93 @@ module WEBrick
 
       assert_equal 0, logger.messages.length
     end
+
+    def test_send_body_io
+      body_r, body_w = IO.pipe
+
+      body_w.write 'hello'
+      body_w.close
+
+      @res.body = body_r
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal 'hello', r.read
+    end
+
+    def test_send_body_string
+      @res.body = 'hello'
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal 'hello', r.read
+    end
+
+    def test_send_body_string_io
+      @res.body = StringIO.new 'hello'
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal 'hello', r.read
+    end
+
+    def test_send_body_io_chunked
+      @res.chunked = true
+
+      body_r, body_w = IO.pipe
+
+      body_w.write 'hello'
+      body_w.close
+
+      @res.body = body_r
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal "5\r\nhello\r\n0\r\n\r\n", r.read
+    end
+
+    def test_send_body_string_chunked
+      @res.chunked = true
+
+      @res.body = 'hello'
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal "5\r\nhello\r\n0\r\n\r\n", r.read
+    end
+
+    def test_send_body_string_io_chunked
+      @res.chunked = true
+
+      @res.body = StringIO.new 'hello'
+
+      r, w = IO.pipe
+
+      @res.send_body w
+
+      w.close
+
+      assert_equal "5\r\nhello\r\n0\r\n\r\n", r.read
+    end
   end
 end