Add back field fCause, initialize and use in the constructor (via ini…

…tCause()) to avoid npe and unused field warnings, respectively

Issue junit-team#1178, junit-team#976

src/main/java/org/junit/internal/ArrayComparisonFailure.java

@@ -16,11 +16,12 @@ public class ArrayComparisonFailure extends AssertionError {
 
     /*
      * We have to use the f prefix until the next major release to ensure
-     * serialization compatibility. 
+     * serialization compatibility.
      * See https://github.com/junit-team/junit4/issues/976
      */
     private final List<Integer> fIndices = new ArrayList<Integer>();
     private final String fMessage;
+    private final AssertionError fCause;
 
     /**
      * Construct a new <code>ArrayComparisonFailure</code> with an error text and the array's
@@ -32,7 +33,8 @@ public class ArrayComparisonFailure extends AssertionError {
      */
     public ArrayComparisonFailure(String message, AssertionError cause, int index) {
         this.fMessage = message;
-        initCause(cause);
+        this.fCause = cause;
+        initCause(fCause);
         addDimension(index);
     }