[Fix] Dot add :// to mailto: URIs (RFC 6068)

[Fix] Dot add :// to mailto: URIs (RFC 6068)

mailto: is non-hierarchical — the // authority component never applies.
The bug was in rspamd_mailto_parse setting RSPAMD_URL_FLAG_MISSINGSLASHES
when // was absent, causing rspamd_url_parse_text to
inject :// into the stored string.

Note: bare email addresses detected via the @ pattern (user@example.net
in text, no scheme prefix) still go through a different path where
"mailto://" is injected as a literal prefix — that's a separate issue
and out of scope here.