res_pjsip_messaging: Allow application/* for in-dialog MESSAGEs

In addition to text/* content types, incoming_in_dialog_request now
accepts application/* content types.

Also fixed a length issue when copying the body text.  It was one
character short.

ASTERISK-27942

Change-Id: I4e54d8cc6158dc47eb8fdd6ba0108c6fd53f2818