path: resolve normalize drive letter to lower case

make path.resolve work the same as path.normalize.

see https://github.com/joyent/node/blob/v0.12/lib/path.js#L180
What is the point of this change? process.cwd() makes no promise of normalizing the result, and I think this change might not be necessary. I'd rather keep it case-preserving unless there is a good reason to change that.