When request URI uses sips scheme, TLS must always be used (thanks Peter Koletzki for the report)

[nanang]

As specified in ​RFC3261 8.1.2:

   Independent
   of which URI is used as input to the procedures of [4], if the
   Request-URI specifies a SIPS resource, the UAC MUST follow the
   procedures of [4] as if the input URI were a SIPS URI.

   Local policy MAY specify an alternate set of destinations to attempt.
   If the Request-URI contains a SIPS URI, any alternate destinations
   MUST be contacted with TLS.

[nanang]

In 4770:

Fixed #1740: TLS will be used whenever request URI uses "sips" scheme regardless the target-URI scheme/transport. This behavior is configurable via 'PJSIP_DONT_SWITCH_TO_TLS' in compile-time, or 'pjsip_cfg()->disable_tls_switch' in run-time.