don't overwrite content-length if its already there

we shouldn't overwrite something that the client likely did it
willlingly

src/kemal/ext/response.cr

@@ -2,7 +2,7 @@ class HTTP::Server::Response
   class Output
     def close
       # ameba:disable Style/NegatedConditionsInUnless
-      if !response.wrote_headers? && !response.headers.has_key?("Content-Range") && !response.headers.has_key?("Content-Length")
+      unless response.wrote_headers? && !response.headers.has_key?("Content-Range") && !response.headers.has_key?("Content-Length")
         response.content_length = @out_count
       end