fix render an image from an action ex: captcha



use case ex: captcha
```erlang
catpcha('GET', [], _ReqCtx) ->
BinImg = getcaptcha(SessionId),
{output, [BinImg], [{"Content-Type","image/png"}]}.
```
Merge pull request #387 from mihawk/patch-7

fix render an image from an action ex: captcha