Default transfer 'dispose stream on completion' option to true instead of false

[jpdillingham]

Reviewing the code and I'm confused why this is defaulted to false in the first place. In the slskd code it's set to true, and there's a note about making sure it's set that way. This seems like objectively the more sane default, as unsuspecting users won't know to flush output streams otherwise and may be truncating downloaded files.

This will be a breaking change.