Define a less strict == for link objects

[Anthony Blaom, PhD]

julia> GLM.LogitLink() == GLM.LogitLink()
false

Probably not the intended behaviour 😄

This also causes problems in MLJ, which wraps some GLM models. MLJ needs to be able to detect when model hyper-parameters change, when determining whether to retrain models (eg, in a pipeline with several components, some of which might change while others do not) - and for other reasons.