CPLEX.jl modifies user's input data

[mlubin]

JuMP's internal variable bounds, when Inf or -Inf, get modified after calling CPLEX.jl, I think due to:
https://github.com/JuliaOpt/CPLEX.jl/blob/6f9c2445a33a93972808ef6d6bbd87bb03b6d272/src/cpx_vars.jl#L8

[joehuchette]

...well loadproblem! does have a bang. Do we want the contract to explicitly forbid altering input data? We should add it to the MPB docs then.

[mlubin]

The bang is meant for the first argument (the model), not the input data. It's generally good behavior to not destroy the user's data in solver interfaces, we can add this to the MPB specification.