Always set content-length

Not setting it makes some clients unhappy.

.release-notes/set-content-length.md

@@ -0,0 +1,3 @@
+## Always set Content-Length
+
+There was a bug where the Content-Length header could end up not being sent. This would cause many clients (such as curl) to hang.

http_server/_test_reponse.pony

@@ -12,7 +12,7 @@ class \nodoc\ iso _BuildableResponseTest is UnitTest
 
   fun apply(h: TestHelper) ? =>
     let without_length = BuildableResponse()
-    h.assert_is[None](None, without_length.header("Content-Length") as None)
+    h.assert_is[String]("0", without_length.header("Content-Length") as String)
 
     let with_length = BuildableResponse(where content_length' = 42)
     h.assert_eq[String]("42", with_length.header("Content-Length") as String)

http_server/response.pony

@@ -298,8 +298,8 @@ class val BuildableResponse is (Response & ByteSeqIter)
     match cl
     | let clu: USize =>
       set_header("Content-Length", cl.string())
-    // | None =>
-    // TODO: drop header
+    | None =>
+      set_header("Content-Length", "0")
     end
     this