Charset setting support

[kirs]

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

[kirs]

@jodosha please tell if the implementation is OK and I'll add the docs then.

[coveralls]

Coverage decreased (-0.75%) when pulling 8cc891b on kirs:charset-setting into 9698719 on lotus:master.

[jodosha]

This should be:

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).

[jodosha]

@kirs Thanks for this PR.

There aren't tests for the following scenarios:

Local override

def call(params)
  self.charset = 'latin1'
end

Configuration

Lotus::Controller.configure do
  default_charset 'ISO-8859-1'
end

[kirs]

@jodosha done!