mod_spandsp fax_header and fax_ident _undef_ not being respected

[MayamaTakeshi]

According to the documentation (https://developer.signalwire.com/freeswitch/FreeSWITCH-Explained/Modules/mod_spandsp_6587021/)
we can suppress the fax header line (txfax) if we set

fax_header=_undef_ 

However, the code in mod_spandsp_fax.c doesn't have code to handle this (neither for fax_ident):

freeswitch/src/mod/applications/mod_spandsp/mod_spandsp_fax.c

Line 1433 in d22aec6

if ((tmp = switch_channel_get_variable(channel, "fax_header"))) {

So the fax is sent with header and ident as:

_undef_ 

The value

_undef_ 

only works in the general configuration of mod_spandsp parameters header and ident:

<configuration name="spandsp.conf" description="FAX application configuration"> <fax-settings> <param name="ident" value="_undef_"/> <param name="header" value="_undef_"/> </fax-settings> </configuration> 

This PR addresses this issue.