res_pjsip_messaging: Update To URI only if it is a SIP(S) URI

res_pjsip_messaging: Update To URI only if it is a SIP(S) URI

When a message is sent via ARI, the ARI endpoint only provides a To
field which is also used as destination field. This means that the To
field might not necessarily contain a SIP URI but might instead specify
an Asterisk endpoint (in MessageDestinationInfo format). This led to
many warnings even though the message was sent correctly.

The fix is to only call `ast_sip_update_to_uri` if the To field starts
with the sip: or sips: scheme.

Resolves: #1357