lxd/networks: Fixes bug in doNetworkUpdate that prevents removal of non-node specific keys
lxd/networks: Fixes bug in doNetworkUpdate that prevents removal of non-node specific keys

This was introduced when the PUT method was merged with the PATCH method.

However there is still sufficiently shared logic to justify putting in a small tweak to handle the different behaviour between PUT and PATCH.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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