Allow passing through streaming bodies.

lib/rack/response.rb

@@ -95,11 +95,23 @@ def chunked?
       CHUNKED == get_header(TRANSFER_ENCODING)
     end
 
+    def enumerable_body?
+      @body.respond_to?(:each)
+    end
+
+    def streaming_body?
+      !enumerable_body?
+    end
+
+    def no_entity_body?
+      enumerable_body? && STATUS_WITH_NO_ENTITY_BODY[@status]
+    end
+
     # Generate a response array consistent with the requirements of the SPEC.
     # @return [Array] a 3-tuple suitable of `[status, headers, body]`
     # which is suitable to be returned from the middleware `#call(env)` method.
     def finish(&block)
-      if STATUS_WITH_NO_ENTITY_BODY[@status]
+      if no_entity_body?
         delete_header CONTENT_TYPE
         delete_header CONTENT_LENGTH
         close

test/spec_response.rb

@@ -638,6 +638,16 @@ def obj.each
     res.body.wont_be :closed?
   end
 
+  it "doesn't clear #body when 101 and streaming" do
+    res = Rack::Response.new
+
+    streaming_body = proc{|stream| stream.close}
+    res.body = streaming_body
+    res.status = 101
+    res.finish
+    res.body.must_equal streaming_body
+  end
+
   it "flatten doesn't cause infinite loop" do
     # https://github.com/rack/rack/issues/419
     res = Rack::Response.new("Hello World")