#1858: restrict collection.Immutable equals to fellow Immutable wrappers

Delegating equals() to the inner collection produced asymmetric equality
when compared with a plain list: 'wrapper.equals(list)' was true (via
delegation) but 'list.equals(wrapper)' was false because Immutable is a
Collection, not a List. The Collection interface adds no stipulations to
Object.equals, so restricting equals() to other Immutable instances
keeps the wrapper symmetric with any plain List/Set while still allowing
two wrappers of equal content to compare equal. hashCode() keeps
delegating to the inner collection — equal objects still have equal
hashCodes; unequal objects may share a hash, which is permitted.

Author: claude

src/main/java/org/cactoos/collection/Immutable.java

@@ -120,6 +120,8 @@ public int hashCode() {
 
     @Override
     public boolean equals(final Object obj) {
-        return this.col.equals(obj);
+        return this == obj
+            || obj instanceof Immutable
+            && this.col.equals(((Immutable<?>) obj).col);
     }
 }