have tests for visualization and ubl convert. Insist in UTF-8 as charset in more cases.

library/src/main/java/org/mustangproject/ZUGFeRD/CustomXMLProvider.java

@@ -18,6 +18,8 @@
  *********************************************************************** */
 package org.mustangproject.ZUGFeRD;
 
+import java.nio.charset.StandardCharsets;
+
 public class CustomXMLProvider implements IXMLProvider {
 
 	protected byte[] zugferdData;
@@ -29,7 +31,7 @@ public class CustomXMLProvider implements IXMLProvider {
 	}
 
 	public void setXML(byte[] newData) {
-		String zf = new String(newData);
+		String zf = new String(newData, StandardCharsets.UTF_8);
 		if (!zf.contains("CrossIndustry")) {
 			throw new RuntimeException("ZUGFeRD XML does not contain (<rsm:)CrossIndustry and can thus not be valid");
 		}

library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRDVisualizer.java

@@ -23,6 +23,7 @@ import javax.xml.transform.stream.StreamResult;
 import javax.xml.transform.stream.StreamSource;
 
 import java.io.*;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Paths;
 import java.nio.file.StandardOpenOption;
@@ -77,7 +78,7 @@ public class ZUGFeRDVisualizer {
 		FileInputStream fis=new FileInputStream(xmlFilename);
 		String fileContent="";
 		try {
-			fileContent = new String(Files.readAllBytes(Paths.get(xmlFilename)));
+			fileContent = new String(Files.readAllBytes(Paths.get(xmlFilename)), StandardCharsets.UTF_8);
 		} catch (IOException e2) {
 			LOG.log(Level.SEVERE, null, e2);
 		}