Can't "go get" branches with capital letters in their name

[seeruk]

Describe the bug
Attempting to go get a branch with capital letters in the name currently results in a HTTP 500. It appears that this is because Athens isn't decoding the branch name before using it. This is outlined in this issue in the golang/go repository.

Error Message
If applicable add error message to help explain your problem or console output enclose in triple back-ticks eg,

$ go get -u bitbucket.org/icelolly/<...>@ICE-163-<...>
go: finding bitbucket.org/icelolly/<...> ICE-163-<...>
go: finding bitbucket.org/icelolly ICE-163-<...>
go: finding bitbucket.org ICE-163-<...>
go get bitbucket.org/icelolly/<...>@ICE-163-<...>: unexpected status (http://athens.<...>/bitbucket.org/icelolly/<...>/@v/ICE-163-<...>.info): 500 Internal Server Error

(Side note here, this is the same thing that happens if the branch doesn't exist, the error could probably be more useful / obvious, like returning a 404 if the branch wasn't found. In this case, the branch definitely exists).

To Reproduce
Steps to reproduce the behavior:

1. Create a branch on Go Modules based project that includes capital letters.
2. Attempt to download that branch via go get.

Expected behavior
A clear and concise description of what you expected to happen.

It to successfully download the branch.

Environment (please complete the following information):

• OS: Linux <...> 5.1.15-arch1-1-ARCH #1 SMP PREEMPT Tue Jun 25 04:49:39 UTC 2019 x86_64 GNU/Linux
• Go version: go version go1.12.6 linux/amd64
• Proxy version: 0.4.0 (I think, I'm using the Helm chart, version 0.2.11, but if you go to the version endpoint of Athens, that just reports "unset" as the version).
• Storage: fs