Changes

SIP_Templates: Always set Content-Length

According to RFC3261 7.4.2, 7.5, 20.14 Content-Length SHOULD be set over
any transport, and MUST be set over TCP.
Hence, make sure we set it in all cases.
This allows Asterisk being able to decode SIP messages sent to it over a
TCP connection, since it uses the Content-Length field to look up for
message boundaries over the TCP stream.

Change-Id: I26e444fe466b4742d74ef5a6e371ce8488a5afcf