Fix possible NPE in method equals (ParameterizedAssertionError)

```java
@Override
public boolean equals(Object obj) {
    return toString().equals(obj.toString());
}
```

Problems:
* As you can see the old version of "equals" method could throw NullPointerException (obj.toSting()). 
* And also it has not got check for correct class (instanceOf).

It will be better to make additional check: obj instanceof ParameterizedAssertionError