Changed
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*   Use only a prefix of the feed to detect encodings,
    instead of reading the whole feed in memory.
    This reduces the memory usage of parse() by up to ~3x (66-73%),
    but may result in the wrong encoding being detected in rare cases;
    use ``feedparser.parse(optimistic_encoding_detection=False)``
    to get the original behavior (read the whole feed in memory).
    (#296, #302)
