Fixed method ambiguity for MALA

[torfjelde]

If we forget to pass initial parameters to MALA, we encounter

AdvancedMH.jl/src/MALA.jl

Line 23 in fb7e872

propose(rng::Random.AbstractRNG, ::MALA, model) = error("please specify initial parameters")

which is currently causing method ambiguities.

I've added both a fix + explicit testing of this scenario.

[mhauru]

The test failure is only on nightly, but it seems it's only a case of Fortune being against us:

   Expression: ≈(mean(chain3.μ), 0.0, atol = 0.1)
   Evaluated: -0.10015010264804329 ≈ 0.0 (atol=0.1)

Unless the current atol is already a safe bound that should never come even close to being violated, may be worth relaxing it a tiny bit.

[mhauru]

LGTM