remove duplicate of PATH environment variable on Windows

[aberkovsky]

What:
Issue #216

Why:
On Windows operating systems, environment variables are case-insensitive.
However, if when you spawn a new process and pass the env with two variables (PATH and Path), in this new process process.env will contains both variables, and this breaks the subsequent logic.

Npm has logic to set both Path and PATH (if exist) variables on Windows, and by default used Path on Windows.
But if I use cgwin terminal on Windows for example, process.env contains PATH.

How:
I add patchPathEnv.js

function patchPathEnv(env) {
  if (isWindows() && env.Path && env.PATH) {
    delete env.Path
  }
}

remove duplicate variable if both exists.

Checklist:

• [N/A] Documentation
• [ x] Tests
• [ x] Ready to be merged
  

[codecov]

Codecov Report

Merging #218 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #218   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      5    +1     
  Lines          78     84    +6     
  Branches       18     19    +1     
=====================================
+ Hits           78     84    +6

Impacted Files	Coverage Δ
src/patch-path.js	100% <100%> (ø)
src/index.js	100% <100%> (ø)	⬆️

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[kentcdodds]

But if I use cgwin terminal on Windows for example, process.env contains PATH.

Does this break for people who aren't using cgwin?

[kentcdodds]

Could you add some comments here to explain why we're doing this because looking at this code as it is is unclear.

[kentcdodds]

#257