XmlStreamReader encoding regexp does not work anymore without version

Hi,

the regular expression for the encoding was changed in XmlStreamReader 
between 2.13.0 and 2.15.1.

It now requires a version attribute in the XML declaration and does not 
work anymore with some real world files.

For example, the encoding from the following example declaration is 
respected by 2.13.0, but not by 2.15.1

<?xml encoding='Cp1047'?>

It works if the version is specified: <?xml version="1.0" 
encoding='Cp1047'?>

However note, that https://www.w3.org/TR/xml/#NT-EncodingDecl also 
mentions examples without version attribute, at least for entities. It 
would be good to restore the previous behavior, IMHO.

Cheers,
Andreas