Mention case-insensitivity in process.env docs

[oliversalzburg]

On Windows OS, environment variables are case-insensitive.

14:46 $ node
> process.env.foo = "lol"
'lol'
> process.env.fOo
'lol'

I think it could be beneficial to make a note about this in the docs.

[oliversalzburg]

@nfischer This behavior is present in Node 4 (and probably earlier as well). We initially noticed this in 4 and assumed it would be something that had changed between 4 and 6. We then realized that it had nothing to do with the version, but with us moving the code from Windows to UNIX.

Also note that this is not really a Node-specific thing. This is simply how Windows handles environment variables.

[nfischer]

I understand this is how Windows does env variables. I just wanted to verify that the NodeJS API for accessing those env variables is likewise case-insensitive (as opposed to not guaranteeing if the key is "PATH" or "Path" or "path"). Case insensitive is the only thing that makes sense to me, but it's nice to have the guarantee 😄

I would hate to rely on this being case-insensitive, only to have it crash on someone else's Windows setup because it was undefined behavior. It would also be nice to know if this is intended as a feature in NodeJS going forward (again, I would hate to rely on this if it's likely to be reversed in v7 or v8).

[oliversalzburg]

@nfischer The code relies on the GetEnvironmentVariableW Windows API function, which already has case-insensitive behavior. I would consider it extremely unlikely that this behavior would ever change.