Accept a negative TTL with purge.soft

lib/libvmod_purge/vmod.vcc

@@ -93,8 +93,8 @@ $Function INT soft(DURATION ttl = 0, DURATION grace = -1, DURATION keep = -1)
 
 Description
         Sets the TTL, grace and keep. By default, TTL is set to 0 with grace
-        and keep periods left untouched. Setting a negative value for any of
-        the parameters leaves them untouched. Setting all three parameters to
+        and keep periods left untouched. Setting a negative value for grace or
+        keep periods leaves them untouched. Setting all three parameters to
         0 is equivalent to a hard purge. It can only be called from ``vcl_hit``
         or ``vcl_miss``. It returns the number of soft-purged objects.
 

lib/libvmod_purge/vmod_purge.c

@@ -49,8 +49,6 @@ vmod_soft(VRT_CTX, VCL_DURATION ttl, VCL_DURATION grace, VCL_DURATION keep)
 {
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
-	if (ttl < 0)
-		ttl = NAN;
 	if (grace < 0)
 		grace = NAN;
 	if (keep < 0)