Add more tests

[abelsiqueira]

I found a small error and decided to create more tests using various constructor options.

[codecov[bot]]

Codecov Report

Merging #17 into master will increase coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #17      +/-   ##
=========================================
+ Coverage    92.3%   92.4%   +0.09%     
=========================================
  Files           3       3              
  Lines          78      79       +1     
=========================================
+ Hits           72      73       +1     
  Misses          6       6

Impacted Files	Coverage Δ
src/qpmodel.jl	93.42% <ø> (+0.08%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update a0d866d...33a763a. Read the comment docs.

[dpo]

Thank you.