feature: added new method "get_stale" to shared dict objects, which returns the value (if not freed yet) even if the key has already expired. thanks Matthieu Tourne for the patch in #249.
updated docs to reflect recent changes.
documented the new "get_stale" method for shdict objects.
added tests to improve the code coverage in file ngx_http_lua_headers_out.c.
Merge branch 'master' into getlifetime