remove stream object from emitted file parts

Hello,

I have encountered a stack overflow exception with the last connect-multiparty module version when using domains. The root cause is a clone function in the qs library used by connect-multiparty trying to clone the ws (WriteStream) object referenced by the file object emitted by node-multiparty.

Once the file stream has been closed, I think it is not needed anymore and we can safely delete it from the emitted file and avoid unhappy interactions. What do you think of it ?

Regards