patch 8.0.0787: cannot send CTRL-W command to terminal job

Problem:    Cannot send CTRL-W command to terminal job.
Solution:   Make CTRL-W . a prefex for sending a key to the job.

src/option.c

@@ -2773,7 +2773,7 @@ static struct vimoption options[] =
     {"termkey", "tk",	    P_STRING|P_ALLOCED|P_RWIN|P_VI_DEF,
 #ifdef FEAT_TERMINAL
 			    (char_u *)VAR_WIN, PV_TK,
-			    {(char_u *)"\x17", (char_u *)NULL}
+			    {(char_u *)"", (char_u *)NULL}
 #else
 			    (char_u *)NULL, PV_NONE,
 			    {(char_u *)NULL, (char_u *)0L}