manager: Fix HTTP connection reference leaks.

Fix reference leak that happens if (session && !blastaway).

ASTERISK-24505 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4153/
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Merged revisions 427641 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427642 f38db490-d61c-443f-a65b-d21fe96a405b