allow returining NGX_OK in mruby

Though we communicated about whether treating NGX_OK like NGX_HTTP_OK in #31, 
ngx_mruby does not works as same as mod_mruby even if 7b5bbab is applied.


After all, do you want to make ngx_mruby work as same as mod_mruby or not allowed NGX_OK in mruby?