Bug fix: swap with temporary is not allowed

This is a valid fix for the issue raised by a-kulbei in https://github.com/a-kulbei/httpxx/commit/46f1372582ab18fdeb1fca11479207fb77c11062 (swapping with a temporary is not allowed).

The fix suggested by a-kulbei is not valid since clearing a string does not deallocate memory buffers (which defeats the purpose of the reset_buffers method all together, one can use clear if we only want to empty the buffer).