res_fax_spandsp: fix t38 transmission bug caused by not returning success

This patch fixes the problem, but the issue includes a test which is still
being considered for the automated test suite.

(issue ASTERISK-20919)
Reported by: NITESH BANSAL
Patches:
	patch_ast_fax_spandsp.patch uploaded by NITESH BANSAL (license 6418)
........

Merged revisions 379949 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@379950 f38db490-d61c-443f-a65b-d21fe96a405b