protoc_rust does not handle file path with backslashes properly on windows.

[dvush]

Hi.

It seems that protoc_rust does not handle slashes in paths correctly.
If you pass valid path on windows(with "") to protoc_rust::run it won't work.

example that does not work on windows https://github.com/dvush/protoc_path_buf.git

[stepancheg]

I've set up travis-ci on Windows: https://travis-ci.org/stepancheg/rust-protobuf/builds/469847927

This is the first step.

Next steps are:

• fix the build
• reproduce and fix the issue you've reported

(You can send PR of course, but I cannot fix it myself without CI, I don't have a Windows box)

[stepancheg]

OK, the problem is reproduced on Windows in CI: https://travis-ci.org/stepancheg/rust-protobuf/jobs/470187677 now need to fix it.

[stepancheg]

Should be fixed now. Published v2.2.2. Please reopen if the problem still exists.