net.http.server: Fix #1789

Unregistering the response before sending the trailer of the chunked
transfer encoding prevents opportunistic writes from being invoked and
running this code again when, which may cause an error when closing the
file handle a second time.

Normally the file size is known, so no chuck headers are sent.

net/http/server.lua

@@ -377,12 +377,12 @@ function _M.send_file(response, f)
 			-- io.write("."); io.flush();
 			response.conn:write(chunk);
 		else
+			incomplete[response.conn] = nil;
 			if chunked then
 				response.conn:write("0\r\n\r\n");
 			end
 			-- io.write("\n");
 			if f.close then f:close(); end
-			incomplete[response.conn] = nil;
 			return response:done();
 		end
 	end