[Fix] lua_url: Re-encode control characters and spaces in URL tostring

[Fix] lua_url: Re-encode control characters and spaces in URL tostring

The URL parser (rspamd_url_decode) decodes percent-encoded sequences
like %20 back to literal characters in the internal representation.
When tostring() returned these decoded URLs, spaces and control chars
would break subsequent re-parsing (e.g., in url_redirector redirect
chains and Redis cache round-trips). Fix by re-encoding characters
<= 0x20 on serialization, matching browser behavior: decode internally
for matching, re-encode on copy.