Purchase partner onchange

Hello, since the new API is not in place in the purchase module, it's essential that existing on_changes pass the context argument so that overriders can properly use the context.

Adding the context argument in the overrider signature is not option as it would make it incompatible with any other module not adding that extra argument.

That is why we need that signature change to pass the context during the v8 18 months or so lifecycle to make 3rd party apps more compatible until the new API is in place is the official addons in v9 or so.

NOTE:  not that this PR depends on https://github.com/odoo/odoo/issues/911 which is a bugfix.
So either you proceed PR #911 before, either both commits in that single merge.

Regards