Fixing Xerces dtd/schema backward compatibility

[the-thing]

Fixes #306

Changes

• data dictionary blocks external dtd/schema properties only when using JDK Xerces implementation
• removed dtd/schema properties from quickfix.Message#toXML(quickfix.DataDictionary) - it is not required as we are neither loading, nor validation against schemas
• unit tests

Comments

• dtd/schema properties probably are not required in org.quickfixj.codegenerator.MessageCodeGenerator as it uses files that belong to the codebase, so changes pointing to external dtd/schema would be visible in PRs (the only reason I can think to keep it there is if somebody is dependent on org.quickfixj:quickfixj-codegenerator directly in their project to customise message generation)
• this change fixes Xerces compatiblity problem, but if external Xerces dependency is being used, there is not easy way to supply javax.xml.parsers.DocumentBuilderFactory with custom properties or features (https://xerces.apache.org/xerces2-j/features.html, https://xerces.apache.org/xerces2-j/properties.html). Additional changes have to be made to propagate javax.xml.parsers.DocumentBuilderFactory across the board.