def content_length
      # http://greenbytes.de/tech/webdav/rfc7230.html#rfc.section.3.3.3
      # Clause 3: "If a message is received with both a Transfer-Encoding
      # and a Content-Length header field, the Transfer-Encoding overrides the Content-Length.
      return nil if @headers.include?(Headers::TRANSFER_ENCODING)

      value = @headers[Headers::CONTENT_LENGTH]
      return nil unless value

      begin
        Integer(value)
      rescue ArgumentError
        nil
      end
    end
def content_length
      # http://greenbytes.de/tech/webdav/rfc7230.html#rfc.section.3.3.3
      # Clause 3: "If a message is received with both a Transfer-Encoding
      # and a Content-Length header field, the Transfer-Encoding overrides the Content-Length.
      return nil if @headers.include?(Headers::TRANSFER_ENCODING)

      value = @headers[Headers::CONTENT_LENGTH]
      return nil unless value

      begin
        Integer(value)
      rescue ArgumentError
        nil
      end
    end
Value of the Content-Length header.

 @return [nil] if Content-Length was not given, or it's value was invalid
   (not an integer, e.g. empty string or string with non-digits).
 @return [Integer] otherwise