Use ReaderFactory.newXmlReader() instead of ISR in MavenHelper

[fgunbin]

plexus-utils's MXParser obtains encoding from XmlReader (see codehaus-plexus/plexus-utils#163) or from ISR.
In our case MavenHelper creates ISR, and it then reports "default" encoding, which most probably will be UTF-8.

When the pom file contains encoding declaration with something else, like ISO-8859-1 in https://repo1.maven.org/maven2/org/apache/commons/commons-parent/48/commons-parent-48.pom, this results in the error:

An error occurred attempting to read POM
org.codehaus.plexus.util.xml.pull.XmlPullParserException: UTF-8 BOM plus xml decl of iso-8859-1 is incompatible (position: START_DOCUMENT seen <?xml version="1.0" encoding="iso-8859-1"... @1:41)
	at org.codehaus.plexus.util.xml.pull.MXParser.parseXmlDeclWithVersion(MXParser.java:3366)
	at org.codehaus.plexus.util.xml.pull.MXParser.parseXmlDecl(MXParser.java:3288)

This PR switches to using plexus-utils's ReaderFactory, which "knows better". This eliminates the above error.

[fgunbin]

Should fix #107

[glefloch]

Thanks for this @fgunbin. Could you add a test that assert no more exceptions are present in logs?

[fgunbin]

@glefloch Added, it fails without my patch, and passes with it.
Thanks.

[glefloch]

Great thanks!

[fgunbin]

@glefloch Thank you!