Merge of r4615: write handler reset in ngx_http_named_location().

Merge of r4615: write handler reset in ngx_http_named_location().

On internal redirects this happens via ngx_http_handler() call, which is
not called on named location redirect.  As a result incorrect write handler
remained (if previously set) and this might cause incorrect behaviour (likely
request hang).

Patch by Yichun Zhang (agentzh).