allow sending request to outbound proxy

[lazedo]

Hi @kalta , not sure if this is the right place to patch.

we would like to send directly to proxy without using Route headers because uri is not resolvable.

Uri = "sip:10102@1.sip-stress.com",
nksip_uac:invite(App, Uri, [{outbound_proxy, "sip:10.21.0.3:7000"}
                           ,contact
                           ,{body, nksip_sdp:new()}
                           ,auto_2xx_ack
                           ]).

[kalta]

Hi. I'm back after this long time... This conflicts now with v0.6, but I will add it

[lazedo]

@kalta welcome back :) and thanks