[core] (data_unset *) from array_get_element_klen

return (data_unset *) from array_get_element_klen() to prep for
putting buffer into data_string for value member, rather than as
(buffer *)

(allow in-place modification of these buffer values)

src/array.c

@@ -142,7 +142,7 @@ static int32_t array_get_index(const array * const a, const char * const k, cons
 }
 
 __attribute_hot__
-const data_unset *array_get_element_klen(const array * const a, const char *key, const size_t klen) {
+data_unset *array_get_element_klen(const array * const a, const char *key, const size_t klen) {
     const int32_t ipos = array_get_index(a, key, klen);
     return ipos >= 0 ? a->data[ipos] : NULL;
 }

src/array.h

@@ -88,7 +88,7 @@ __attribute_pure__
 int array_is_kvstring(const array *a);
 
 __attribute_pure__
-const data_unset *array_get_element_klen(const array *a, const char *key, size_t klen);
+data_unset *array_get_element_klen(const array *a, const char *key, size_t klen);
 
 __attribute_cold__
 __attribute_pure__

src/http_header.c

@@ -92,8 +92,8 @@ static inline void http_header_token_append(buffer * const vb, const char * cons
 
 __attribute_pure__
 static inline buffer * http_header_generic_get_ifnotempty(const array * const a, const char * const k, const size_t klen) {
-    const data_string * const ds =
-      (const data_string *)array_get_element_klen(a, k, klen);
+    data_string * const ds =
+      (data_string *)array_get_element_klen(a, k, klen);
     return ds && !buffer_string_is_empty(ds->value) ? ds->value : NULL;
 }