Fix mid using specialized version for alpha=0.5

[dpsanders]

No description provided.

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   92.14%   92.19%   +0.04%     
==========================================
  Files          22       22              
  Lines         917      922       +5     
==========================================
+ Hits          845      850       +5     
  Misses         72       72

Impacted Files	Coverage Δ
src/multidim/intervalbox.jl	81.25% <ø> (ø)	⬆️
src/intervals/arithmetic.jl	100% <100%> (ø)	⬆️

---

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 c0bb130...c9f9f2d. Read the comment docs.

[lbenet]

@dpsanders It seems that I must re-tag; sorry for the mess.
What I want to bring up is that then we could merge the couple of commits I somehow forgot (this one, #30, and maybe #22) and before the new tag.