Fix whitespace handling in `X-Forwarded-For` header parsing

[adam-p]

Fixes #227

Summary of Changes

1. Split X-Forwarded-For using space rather than comma-space.
2. Trim whitespace from resulting value.
3. Add tests. (Including for Forwarded parsing, which didn't have this problem.)

[adam-p]

I don't know how to interpret the CircleCI failure.

[amustaque97]

@adam-p thank you for working on this. In order to fix the pipeline issue. You need to add line //go:build go1.8 at the top of the file handlers_go18_test.go in the project.

[coreydaley]

@adam-p If you would resolve the conflict with this pull request we can get it reviewed and merged. Thanks!