Fix Mi metric

[Luni-4]

The new Halstead implementation has broken the Mi computation, this PR fixes it. Added a test to simplify catching these errors also.

Thanks in advance for your review! :)

[codecov-commenter]

Codecov Report

Merging #258 into master will increase coverage by 0.05%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #258      +/-   ##
==========================================
+ Coverage   17.32%   17.37%   +0.05%     
==========================================
  Files          46       46              
  Lines        5918     5923       +5     
  Branches      698      699       +1     
==========================================
+ Hits         1025     1029       +4     
  Misses       4490     4490              
- Partials      403      404       +1     

Impacted Files	Coverage Δ
src/metrics/mi.rs	60.60% <66.66%> (+0.60%)	⬆️
src/spaces.rs	59.73% <100.00%> (+0.54%)	⬆️

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