Fix bug in relax_integrality

[odow]

relax_integrality doesn't work for solvers in direct mode not supporting VariablePrimalStart:

julia> model = direct_model(GLPK.Optimizer())
A JuMP Model
Feasibility problem with:
Variables: 0
Model mode: DIRECT
Solver name: GLPK

julia> @variable(model, x, Int)
x

julia> optimize!(model)

julia> undo = JuMP.relax_integrality(model)
ERROR: ArgumentError: ModelLike of type GLPK.Optimizer does not support accessing the attribute MathOptInterface.VariablePrimalStart()
Stacktrace:
  [1] get_fallback(model::GLPK.Optimizer, attr::MathOptInterface.VariablePrimalStart, args::MathOptInterface.VariableIndex)
    @ MathOptInterface ~/.julia/packages/MathOptInterface/YDdD3/src/attributes.jl:296
  [2] get(model::GLPK.Optimizer, attr::MathOptInterface.VariablePrimalStart, args::MathOptInterface.VariableIndex)
    @ MathOptInterface ~/.julia/packages/MathOptInterface/YDdD3/src/attributes.jl:293
  [3] get(model::Model, attr::MathOptInterface.VariablePrimalStart, v::VariableRef)
    @ JuMP ~/.julia/packages/JuMP/b3hGi/src/JuMP.jl:1234
  [4] start_value(v::VariableRef)
    @ JuMP ~/.julia/packages/JuMP/b3hGi/src/variables.jl:911
  [5] _info_from_variable(v::VariableRef)
    @ JuMP ~/.julia/packages/JuMP/b3hGi/src/variables.jl:1247
  [6] #34
    @ ~/.julia/packages/JuMP/b3hGi/src/variables.jl:1334 [inlined]
  [7] iterate
    @ ./generator.jl:47 [inlined]
  [8] _collect(c::Vector{VariableRef}, itr::Base.Generator{Vector{VariableRef}, JuMP.var"#34#35"}, #unused#::Base.EltypeUnknown, isz::Base.HasShape{1})
    @ Base ./array.jl:691
  [9] collect_similar
    @ ./array.jl:606 [inlined]
 [10] map
    @ ./abstractarray.jl:2294 [inlined]
 [11] relax_integrality(model::Model)
    @ JuMP ~/.julia/packages/JuMP/b3hGi/src/variables.jl:1333
 [12] top-level scope
    @ REPL[13]:1

[codecov]

Codecov Report

Merging #2666 (4d30472) into master (fe1f0d1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2666   +/-   ##
=======================================
  Coverage   93.51%   93.51%           
=======================================
  Files          44       44           
  Lines        5549     5551    +2     
=======================================
+ Hits         5189     5191    +2     
  Misses        360      360           

Impacted Files	Coverage Δ
src/variables.jl	98.78% <100.00%> (+<0.01%)	⬆️

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