fix bug in colgen when subproblem is infeasible or unbounded

[guimarqu]

closes #747

[codecov]

Codecov Report

Base: 86.87% // Head: 86.90% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (90895b7) compared to base (a68fcf8).
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #752      +/-   ##
==========================================
+ Coverage   86.87%   86.90%   +0.03%     
==========================================
  Files          59       59              
  Lines        5424     5423       -1     
==========================================
+ Hits         4712     4713       +1     
+ Misses        712      710       -2     

Impacted Files	Coverage Δ
src/Algorithm/utilities/helpers.jl	64.28% <ø> (+4.28%)	⬆️
src/MathProg/formulation.jl	81.48% <ø> (+0.21%)	⬆️
src/Algorithm/colgen.jl	86.76% <33.33%> (+0.02%)	⬆️

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