Add buffer to response.

1. add some write methods to response.
2. add test status methods to response.
3. stop providing `WriteHeader()` method.
4. add `FlushOnError` config.
5. separate mux's `HandleStatic`() to `HandleStaticDir()` and
`HandleStaticFile()`.
6. fix a bug of request's `IP()` method, returns '[' on local machine.