The previous implementation would unescape '&', '=', '+', and '%'. The
first three will break the URL when unescaped in the query string, and
the last will break when unescaped anywhere.
The code is now changed to treat the path, query, and fragment parts of
the URL separately when unescaping. We also escape any unsafe characters
and ensure all percent-encodings use uppercase hexits.
And since the old name is no longer accurate,
Parser::replaceUnusualEscapes is deprecated in favor of