StringRenderer xml-encode leads to invalid XML

[tkalmar]

StringRenderer xml-encode leads to invalid xml

for example the String 🩳 leads to the XML-Entity: �� which is according to XML-Spec (https://www.w3.org/TR/REC-xml/#NT-Char) not in the range of allowed chars for XML (Note the ranges differ between XML 1.0 and XML 1.1)
This should either be:

• fixed (don't know what would be the right way?) perhaps raising an error or rejecting the input? Filtering offending chars would also be an option
• documented

Im not shure if for an UTF-8 encoded XML document the encoding of <, >," and ' would be sufficent and all other characters should be passed through

[parrt]

Looks like it might be not hex-encoding the values.

[tkalmar]

No it is hex encoding the value, but the 🩳 is in its encoded form (��) not valid for XML (see the linked spec) I'm not sure if it is legal in unencoded form for UTF-8 encoded xml files

[Clashsoft]

The emoji is encoded in the Java String as two chars due to the code point being over the two byte limit, hence the strange encoded form. It's called a surrogate pair. The fix would be to loop over the code points instead of the characters of the string. I'll make a PR in a bit.