sso_proxy: fix request signer hash panic

[Jusshersmith]

Problem

Occasional panics (http: panic serving 172.17.0.16:49478: d.nx != 0) which seem to be caused by sso_proxy using the same hasher for multiple requests, creating a race condition between the hasher.Reset() and hasher.Write([]byte(repr)) calls.

Solution

Instead of storing a hash.Hash object within the RequestSigner, instead store a function which creates a new hash.Hash. This lets each request be signed using a different Hash.

[jphines]

🤦‍♂

[codecov]

Codecov Report

Merging #274 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
- Coverage   62.21%   62.18%   -0.03%     
==========================================
  Files          54       54              
  Lines        4200     4197       -3     
==========================================
- Hits         2613     2610       -3     
  Misses       1399     1399              
  Partials      188      188

Impacted Files	Coverage Δ
internal/proxy/request_signer.go	84.84% <100%> (-0.66%)	⬇️

[Jusshersmith]

Rebased and removed go.mod and go.sum changes, now that #273 is merged.

[Jusshersmith]

I think I've said the word 'hash' a few too many times now 🤔