[configuration] XMLConfiguration ignore a specific encoding in XML declaration

I found that usign ConfigurationFactory, XMLConfiguration can't recognize a
specific encoding in XML declaration because of calling method
"load(Reader in)" anytime, so I can use only one encoding depend on OS through
all xml files.

After finding this problem, I modified implementation to create
org.w3c.dom.Document by calling
"DocumentBuilder#parse(new InputSource(Inputstrem in))" to avoid characters
garbled (gobbledygook), and then I confirmed to be recoginzed encoding.

I suggest to correct methods which inner class FileConfigurationDelegate in
XMLConfiguration change to override "load(Inputstrem in)", and
call "XMLConfiguration.this.load(Inputstrem in);", and method
"load(InputStream in)" in XMLConfiguration change to call
"DocumentBuilder#parse(new InputSource(Inputstrem in))".