Bump rack-proxy from 0.6.3 to 0.6.4
Bump rack-proxy from 0.6.3 to 0.6.4

Bumps [rack-proxy](https://github.com/ncr/rack-proxy) from 0.6.3 to 0.6.4.
- [Commits](https://github.com/ncr/rack-proxy/compare/v0.6.3...v0.6.4)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
alphagov/content-tagger,alphagov/content-tagger,alphagov/content-tagger