Improve handling of categoryCode

Charge returns the local field if is set.
IZUGFeRDExportableProduct: Use compareTo instead of equals to check the mathematical equality.

library/src/main/java/org/mustangproject/Charge.java

@@ -82,6 +82,13 @@ public class Charge implements IZUGFeRDAllowanceCharge {
 		return true;
 	}
 
+	@Override
+	public String getCategoryCode() {
+		if(categoryCode != null){
+		    return categoryCode;
+		}
+		return IZUGFeRDAllowanceCharge.super.getCategoryCode();
+	}
 
 
 	public Charge setCategoryCode(String categoryCode) {

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

@@ -119,7 +119,7 @@ public interface IZUGFeRDExportableProduct {
 		    return TaxCategoryCodeTypeConstants.INTRACOMMUNITY;// "K"; // within europe
 		} else if (isReverseCharge()) {
 		    return TaxCategoryCodeTypeConstants.REVERSECHARGE;// "AE"; // to out of europe...
-		} else if (getVATPercent().equals(BigDecimal.ZERO)) {
+		} else if (getVATPercent().compareTo(BigDecimal.ZERO) == 0) {
 		    return TaxCategoryCodeTypeConstants.ZEROTAXPRODUCTS; // "Z"; // zero rated goods
 		} else {
 		    return TaxCategoryCodeTypeConstants.STANDARDRATE;// "S"; // one of the "standard" rates (not