Update keepStrings flag behavior

[stleary]

XMLParserConfiguration.withKeepStrings(true) should set the keepStrings flag. It should be cleared if
withKeepNumberAsString(false) or withKeepBooleanAsString(false) is called.
It can be reset to true if both withKeepNumberAsString(true) or withKeepBooleanAsString(true) are called.

This is just to keep isKeepStrings() consistent.

[stleary]

Closed tue to fixed in #968