fix: noop mutator don't overwrite session headers

[davidspek]

This PR changes the noop mutator so that it doesn't overwrite the current session headers with the request headers.

When using the remote_json authorizer, it is possible to configure headers returned by the remote authorizer that will be forwarded to upstream services with the forward_response_headers_to_upstream field. This effectively allows the authorizer to also act as a mutator. Since rules must contain a mutator, if no further mutations need to be performed you would specify the noop mutator. However, since the noop mutator is setting the session headers to be equal to the request headers, this effectively overwrites the headers set by the remote_json authorizer.

Related issue(s)

Checklist

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• I have referenced an issue containing the design document if my change
  introduces a new feature.
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[codecov]

Codecov Report

Merging #1091 (887bdc2) into master (0a767e7) will decrease coverage by 0.08%.
The diff coverage is 50.00%.

❗ Current head 887bdc2 differs from pull request most recent head 206e1e1. Consider uploading reports for the commit 206e1e1 to get more accurate results

@@            Coverage Diff             @@
##           master    #1091      +/-   ##
==========================================
- Coverage   78.24%   78.17%   -0.08%     
==========================================
  Files          80       80              
  Lines        3843     3853      +10     
==========================================
+ Hits         3007     3012       +5     
- Misses        564      566       +2     
- Partials      272      275       +3     

Impacted Files	Coverage Δ
pipeline/mutate/mutator_noop.go	69.56% <50.00%> (-15.06%)	⬇️

[davidspek]

Friendly ping @zepatrik maybe you can also have a look here.

[davidspek]

Ping @aeneasr @zepatrik

[aeneasr]

Sorry for the long review, this LGTM!

[davidspek]

No problem at all :)