Bugfix for join equality when default join type is used

Since http://trac.propelorm.org/changeset/1959#file1 there is a bug in checking equality between two joins if you skip joinType parameter so below won't work:
```php
$j1 = new Join('foo', 'bar');
$j2 = new Join('foo', 'bar', 'INNER JOIN');
$this->assertTrue($j1->equals($j2));
```

The change to bugfix this is simple, I've also added tests for other, currently working test cases.