class Nokogiri::HTML4::Document

def meta_encoding

then nil is returned.
Get the meta tag encoding for this document. If there is no meta tag,
## 
def meta_encoding
  if (meta = at_xpath("//meta[@charset]"))
    meta[:charset]
  elsif (meta = meta_content_type)
    meta["content"][/charset\s*=\s*([\w-]+)/i, 1]
  end
end