Enable IDN2_USE_STD3_ASCII_RULES to idn2 conversion functions
libidn2 defaults to UseSTD3ASCIIRules=false. That allows arbitrary ASCII characters to show up in the toASCII output, including space and underscore. Enable IDN2_USE_STD3_ASCII_RULES to the libidn2 conversion to disallow additional characters from the conversion (see Validity Criteria[1]).
Closes #1610 (closed)
Edited by Ondřej Surý