ParamOverrides seem to be broken

Surely, `ParamOverrides` shouldn't be allow this?

``` python
import param

class Foo(param.ParameterizedFunction):
a = param.String('bar')
b = param.Boolean()

def __call__(self, **params):
print param.ParamOverrides(self,params)
```

``` python
>>> Foo(a=slice, b=3)
{'a': , 'b': 3} overriding params from Foo(a='bar', b=False, name='Foo')
```

Shouldn't the types be checked? Otherwise what is the point of `ParamOverrides`?

Maybe I am just getting confused and I just don't understand what `ParamOverrides` are for...