convert all eltypes of GlmResp args to the eltype of the first

[dsweber2]

min example: GLM.GlmResp(randn(10), Binomial(), LogitLink(), randn(Float32, 10), randn(10)) Gets stuck in a loop because float won't convert Float64 and Float32 to the same type. I just turned the first one into a float, and then made the rest match that type of float explicitly.

[dsweber2]

digging a bit further, it seems the problem I was trying to address with this was actually caused in Lasso.jl, which was inconsistent about handing Float32 and Float64 to GLM.jl. I think this pull is still worth changing, but there are several changes elsewhere that are also needed if the input is somewhat inconsistent about the type of float involved.

[nalimilan]

Thanks. Though note that float only makes a copy when necessary, contrary to what would happen with this PR. Can you change it to use convert instead? Also better use promote_type to choose the appropriate destination type.