Have rekey update the keys of the distribution parameters to match the KeyedDistribution

[bencottier]

Noted in #15:

If the parent distribution is constructed with a KeyedArray but the KeyedDistribution is given a new set of keys there is no expectation that these should match, e.g.

ka = KeyedArray(ones(3); id=[:a, :b, :c])

kd = KeyedDistribution(MvNormal(ka), [:x, :y, :z])

I don't think this explicitly goes against documentation, so not a bug. Maybe against the principles of the package - AxisKeys and AxisSets don't allow inconsistent keys, but there isn't a direct analogy to this, where some inner type (array) and the outer type should have the same keys but they are able to differ.

In any case, it would be useful to check. We could just add the option for rekey to update the "inner" keys to match the "outer" keys.