`MVector` type yields `ImmutableDiffResult`

Hello, I'm using this package through LsqFit.jl. I have a 3d parameter vector, and would like to speed up the code by leveraging StaticArrays.jl, which basically come for free by passing a StaticVector instead of a Vector. In order to support in-place operations, we can further pass an MVector, i.e. a modifiable statically sized array. However, as seen below, Static Arrays always lead to ImmutableDiffResults.

https://github.com/JuliaDiff/DiffResults.jl/blob/80f085ebddbe2cabd691a61cb56cc1fd7fccd309/src/DiffResults.jl#L31-L52

I've done the following overloads to make it work for my case:

DiffResult(value::MArray, derivs::Tuple{Vararg{MArray}}) = MutableDiffResult(value, derivs)
DiffResult(value::Union{Number, AbstractArray}, derivs::Tuple{Vararg{MVector}}) = MutableDiffResult(value, derivs)

I think this is a useful feature and currently can be considered a bug. If there's interest in merging this I can also open a PR.

Start with src/DiffResults.jl lines 31-52 and reproduce the reported behavior using an MVector or MArray, as described through LsqFit.jl and StaticArrays.jl. Check whether the existing merged pull request changed this dispatch, then verify that the reported mutable input produces a MutableDiffResult without affecting existing cases.