SAX parser failure test case.

Hello,

I stumbled upon extremely weird error when using SAX parser. I added a test case, perhaps you'll be able to shed some light on what is going on here. Array intersection seems to be doing something odd here, but I see no reason why this should be happening.

This passes:

```xml
<?xml version="1.0"?><abcd><abcd/></abcd>
```

This also passes:
```xml
<?xml version="1.0"?><ab><a/></ab>
```

This fails:

```xml
<?xml version="1.0"?><abc><abcd/></abc>
```

This also fails:

```xml
<?xml version="1.0"?><abc><ab/></abc>
```

Everything passes if I remove array intersection inside ```start_element```.  (wat)