Automatic promotion with shift_constant

[blegat]

This is needed to make this work at the JuMP level:

julia> @constraint(model, im * x == 1)
ERROR: MethodError: no method matching shift_constant(::MathOptInterface.EqualTo{Float64}, ::ComplexF64)
Closest candidates are:
  shift_constant(::Union{MathOptInterface.EqualTo{T}, MathOptInterface.GreaterThan{T}, MathOptInterface.LessThan{T}}, ::T) where T at ~/.julia/packages/MathOptInterface/IQWV4/src/Utilities/sets.jl:38
  shift_constant(::MathOptInterface.Interval, ::Any) at ~/.julia/packages/MathOptInterface/IQWV4/src/Utilities/sets.jl:49
  shift_constant(::MathOptInterface.Test.UnknownScalarSet, ::Any) at ~/.julia/packages/MathOptInterface/IQWV4/src/Test/test_model.jl:14
Stacktrace:
 [1] build_constraint(_error::Function, expr::GenericAffExpr{ComplexF64, VariableRef}, set::MathOptInterface.EqualTo{Float64})
   @ JuMP ~/.julia/dev/JuMP/src/macros.jl:574
 [2] macro expansion
   @ ~/.julia/dev/JuMP/src/macros.jl:816 [inlined]
 [3] top-level scope
   @ REPL[15]:1