Charset setting support

Rails returns "Content-Type: text/html; charset=utf-8"
and for Lotus it’s just "Content-Type: text/html"
it may be a problem with some languages like Russian
This should be:

```ruby
def charset
  @charset || default_charset || DEFAULT_CHARSET
end

def default_charset
  configuration.default_charset
end
```

So the priority will be:
1. Local override (`self.charset=`)
2. Framework configuration (`Lotus::Controller.configure { default_charset 'x' }`)
3. Default charset (`DEFAULT_CHARSET`).