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.