Better handle explicit chunked encoding responses (#15092)

spec/std/http/server/response_spec.cr

@@ -76,6 +76,15 @@ describe HTTP::Server::Response do
     io.to_s.should eq("HTTP/1.1 304 Not Modified\r\nContent-Length: 5\r\n\r\n")
   end
 
+  it "allow explicitly configuring a `Transfer-Encoding` response" do
+    io = IO::Memory.new
+    response = Response.new(io)
+    response.headers["Transfer-Encoding"] = "chunked"
+    response.print "Hello"
+    response.close
+    io.to_s.should eq("HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\n\r\n5\r\nHello\r\n0\r\n\r\n")
+  end
+
   it "prints less then buffer's size" do
     io = IO::Memory.new
     response = Response.new(io)

src/http/server/response.cr

@@ -255,7 +255,9 @@ class HTTP::Server
       private def unbuffered_write(slice : Bytes) : Nil
         return if slice.empty?
 
-        unless response.wrote_headers?
+        if response.headers["Transfer-Encoding"]? == "chunked"
+          @chunked = true
+        elsif !response.wrote_headers?
           if response.version != "HTTP/1.0" && !response.headers.has_key?("Content-Length")
             response.headers["Transfer-Encoding"] = "chunked"
             @chunked = true
@@ -289,7 +291,7 @@ class HTTP::Server
         status = response.status
         set_content_length = !(status.not_modified? || status.no_content? || status.informational?)
 
-        if !response.wrote_headers? && !response.headers.has_key?("Content-Length") && set_content_length
+        if !response.wrote_headers? && !response.headers.has_key?("Transfer-Encoding") && !response.headers.has_key?("Content-Length") && set_content_length
           response.content_length = @out_count
         end