ARI: Fix crash if integer values used in JSON payload 'variables' object.

Sending the following ARI commands caused Asterisk to crash if the JSON
body 'variables' object passes values of types other than strings.

POST /ari/channels
POST /ari/channels/{channelid}
PUT /ari/endpoints/sendMessage
PUT /ari/endpoints/{tech}/{resource}/sendMessage

* Eliminated RAII_VAR usage in ast_ari_channels_originate_with_id(),
ast_ari_channels_originate(), ast_ari_endpoints_send_message(), and
ast_ari_endpoints_send_message_to_endpoint().

ASTERISK-24751 #close
Reported by:  jeffrey putnam

Review: https://reviewboard.asterisk.org/r/4447/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432404 65c4cc65-6c06-0410-ace0-fbb531ad65f3