Use correct variable when setting content length

None is stringable so the previous code here would work, but, in
theory could eventually lead to "badness". The intent of the code
was to match on a USize | None and set the content length to the
string version of the USize if a USize.

However, it was setting to the string version of USize | None.
Given the match statement, this was fine for now, but, the intent
was clouded.

This could lead to a bug in the future if someone changed the code.

http_server/response.pony

@@ -297,7 +297,7 @@ class val BuildableResponse is (Response & ByteSeqIter)
     _content_length = cl
     match cl
     | let clu: USize =>
-      set_header("Content-Length", cl.string())
+      set_header("Content-Length", clu.string())
     // | None =>
     // TODO: drop header
     end