Interpreter does not fail on context-less ObjectVariable matching

```
the interpreter does not fail on the matching of a typed ObjectVariable without 
any context. even worse: it behaves like it matched successfully.

see 
'/org.storydriven.storydiagrams.interpreter.tests/resources/patterns/MatchingSto
ryNode/MatchingFails.sdm'
```

Original issue reported on code.google.com by `aljoschability` on 23 Jul 2012 at 10:55
