取得したコンテンツの文字列（読み取り専用）
If the object has not finished downloading the data, it will return an empty string.
Use isDone or yield to see if the data is available.
This function expects the web page contents in UTF-8 or ASCII character set. The returned
string might be not correct for other characters or binary data. Use bytes property to
get raw byte array in these cases.
関連項目: bytes プロパティー