Limit req: fix of rbtree node insertion on hash collisions.

The rbtree used in ngx_http_limit_req_module has two level of keys, the top is
hash, and the next is the value string itself. However, when inserting a new
node, only hash has been set, while the value string has been left empty.

The bug was introduced in r4419 (1.1.14).

Found by Charles Chen.