Fix bug in CompositeException.getRootCause (#6380)

* Fix bug in CompositeException.getRootCause 

The original code use to be `if (root == null || root == e)`, but apparently after some refactoring it ended up as `if (root == null || cause == root)`, which I believe is a bug.

This method should probably be `static` (that would have prevented the bug).

* Update unit tests for CompositeException.getRootCause

src/main/java/io/reactivex/exceptions/CompositeException.java

@@ -280,7 +280,7 @@ public int size() {
      */
     /*private */Throwable getRootCause(Throwable e) {
         Throwable root = e.getCause();
-        if (root == null || cause == root) {
+        if (root == null || e == root) {
             return e;
         }
         while (true) {

src/test/java/io/reactivex/exceptions/CompositeExceptionTest.java

@@ -364,7 +364,22 @@ public synchronized Throwable getCause() {
             }
         };
         CompositeException ex = new CompositeException(throwable);
-        assertSame(ex, ex.getRootCause(ex));
+        assertSame(ex0, ex.getRootCause(ex));
+    }
+
+    @Test
+    public void rootCauseSelf() {
+        Throwable throwable = new Throwable() {
+
+            private static final long serialVersionUID = -4398003222998914415L;
+
+            @Override
+            public synchronized Throwable getCause() {
+                return this;
+            }
+        };
+        CompositeException tmp = new CompositeException(new TestException());
+        assertSame(throwable, tmp.getRootCause(throwable));
     }
 }