Changed code to use specific XPathFactoryImpl

[dswitzer]

This changes the XPathFactory call to use a specific XPathFactoryImpl to avoid issues where an environment may be using a default implementation that's incompatible with the code (such as using an outdated Saxon parser). This change attempts to use the default Java implementation, but if that is unavailable will fallback to try to load the default.

This will also help to ensure the XML is always parsed in a consistent manor, since everyone should be using the same XPathFactoryImpl.

This should resolve Issue #191.

[coveralls]

Coverage decreased (-0.1%) to 94.841% when pulling 2311853 on dswitzer:patch-1 into 54b2936 on onelogin:master.

[siggemannen]

This change seem to have some side-effects when running OpenSaml under Karaf-container.
The call
factory = XPathFactory.newInstance(XPathFactory.DEFAULT_OBJECT_MODEL_URI, "com.sun.org.apache.xpath.internal.jaxp.XPathFactoryImpl", java.lang.ClassLoader.getSystemClassLoader());

fails under JDK (1.8.101) with:

java.lang.IllegalAccessError: tried to access method javax.xml.xpath.XPathFactoryFinder.createInstance(Ljava/lang/String;)Ljavax/xml/xpath/XPathFactory; from class javax.xml.xpath.XPathFactory
at javax.xml.xpath.XPathFactory.newInstance(Unknown Source)

Likely due to weird classloading that Karaf likes to do.

I don't see a way to avoid it, short of patching the Util class or bumping JDK-version.
The following call works fine (and returns the same internal class com.sun.org.apache.xpath.internal.jaxp.XPathFactoryImpl)

XPathFactory.newInstance(XPathFactory.DEFAULT_OBJECT_MODEL_URI);

Any workaround suggestions?