This modifies the Document class to use Optional<String> for the
encoding. If the encoding is unknown, the Optional will not have a
value. It also implements the has_encoding() and encoding_or_default()
instance methods, the latter of which will return "UTF-8" as a fallback
if no encoding is present.
The usage of Optional<String> instead of the null string is part of an
effort to explicitly indicate that a string could not have a value.
This also modifies the former callers of encoding() to use
encoding_or_default(). Furthermore, the encoding will now only be set if
it is actually known, rather than just guessed by earlier code.
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