Fix QR based fitting

[andreasnoack]

Avoid erroring out for low rank design matrices when dropcollinear=false.
Avoid unnecessary triangular solves. Avoid indexing in the Q.
Avoid slicing R matrix in a way that triggers a minimum norm solution.
Remove unnecessary temporaries.

#558

I removed the white space so please review with https://github.com/JuliaStats/GLM.jl/pull/559/files?w=1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.71%. Comparing base (e2f6c98) to head (35c3e94).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
- Coverage   89.86%   89.71%   -0.16%     
==========================================
  Files           8        8              
  Lines        1125     1108      -17     
==========================================
- Hits         1011      994      -17     
  Misses        114      114              

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