Introduced njs_chb_append_str() as inline function.

To catch erroneous passing of njs_string_prop_t (instead of njs_str_t)
in compile time.
diff --git a/src/njs_chb.h b/src/njs_chb.h
index 641547c..8e3521f 100644
--- a/src/njs_chb.h
+++ b/src/njs_chb.h
@@ -43,9 +43,6 @@
 #define njs_chb_append_literal(chain, literal)                               \
     njs_chb_append0(chain, literal, njs_length(literal))
 
-#define njs_chb_append_str(chain, str)                                       \
-    njs_chb_append0(chain, (const char *) (str)->start, (str)->length)
-
 
 #define njs_chb_node_size(n) (size_t) ((n)->pos - (n)->start)
 #define njs_chb_node_room(n) (size_t) ((n)->end - (n)->pos)
@@ -61,6 +58,13 @@
 }
 
 
+njs_inline void
+njs_chb_append_str(njs_chb_t *chain, njs_str_t *str)
+{
+    njs_chb_append0(chain, (const char *) str->start, str->length);
+}
+
+
 njs_inline int64_t
 njs_chb_size(njs_chb_t *chain)
 {