http_response: reduce size of multi-line header path

This should save over 100 bytes of bytecode overhead due to
reduced method dispatch points.  The performance difference
when this is actually hit could go either way depending on
how String#<< and realloc(3) interact, but it's uncommon
enough that nobody is expected to notice either way.

lib/unicorn/http_response.rb

@@ -39,7 +39,7 @@ def http_response_write(socket, status, headers, body,
         else
           if value.include?("\n".freeze)
             # avoiding blank, key-only cookies with /\n+/
-            buf << value.split(/\n+/).map! { |v| "#{key}: #{v}\r\n" }.join
+            value.split(/\n+/).each { |v| buf << "#{key}: #{v}\r\n" }
           else
             buf << "#{key}: #{value}\r\n"
           end