Prevent XEE by hiding custom entities by using single quote to
declare a non-UTF-8 encoding.
XML standard, https://www.w3.org/TR/xml/#NT-EncodingDecl, allows single
quote to declare encoding, but we did not support it. Instead, we
incorrectly fell back on the default of UTF-8. That incorrectly kept the
XML as non-UTF-8, and thus prevented our regexp-based custom entity
detection mechanism to work.
Adrien Crivelli