Potential bug fix for nest.Projection._convergent_connect with non-standard receptor types

I think this may be a bug in PyNN but it could be just that my generated NEST cell classes are not connecting into it in the right way.

The projections I created in nest using `nest.Projection_convergent_connect` with non-standard receptor types (for which PyNN uses the `nest.Connect` method instead of `nest.ConvergentConnect` to allow more flexibility in setting receptor parameters) were not returning any connections when they were queried with `nest.GetConnections` (in order to populate the `Projection.connections` property) because the synapse_model used in the filter was `self.nest_synapse_model` instead of 'static_synapse', which the projection was created with. To "fix" this I added the kwarg `model=self.nest_synapse_model` to the `nest.Connect` call and it seems to be working fine now.

Was this the right thing to do?

PS sorry for the poor grammar in my commit message (and probably this one too), it is getting towards the end of the day and I am starting to feel it! :)