fix(core): headers set by the sdk must respect case insensitive keys

[andersfylling]

I noticed that the "Accept" field is added when it is not already present in the request header. However, the check is case sensitive, while the http RFC states that the http header are case insensitive.

This causes the application/json to be added when a user specify accept and not Accept:

    nock(mockBaseUrl)
      .get(new RegExp('/documents/preview'))
      .matchHeader('Accept', 'image/png')
      .query({ documentId: 1 })
      .once()
      .reply(200);

await this.get<ResponseType>(uri, {
      responseType: HttpResponseType.ArrayBuffer,
      headers: {
        accept: 'image/png',
      },
    });

The above test will fail as the accept header contains both image/png and application/json.

[codecov]

Codecov Report

Merging #627 (36e84f8) into master (29b3a17) will decrease coverage by 1.38%.
The diff coverage is 70.62%.

❗ Current head 36e84f8 differs from pull request most recent head b0b8a33. Consider uploading reports for the commit b0b8a33 to get more accurate results

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   77.15%   75.76%   -1.39%     
==========================================
  Files          87       90       +3     
  Lines        2779     3202     +423     
  Branches      390      443      +53     
==========================================
+ Hits         2144     2426     +282     
- Misses        599      776     +177     
+ Partials       36        0      -36     

Impacted Files	Coverage Δ
packages/stable/src/__tests__/testUtils.ts	100.00% <ø> (ø)
packages/stable/src/types.ts	100.00% <ø> (ø)
.../beta/src/api/templateGroups/templateGraphQlApi.ts	40.00% <14.28%> (-26.67%)	⬇️
packages/stable/src/api/3d/nodes3DApi.ts	81.81% <20.00%> (-18.19%)	⬇️
packages/stable/src/api/projects/projectsApi.ts	78.94% <20.00%> (-21.06%)	⬇️
packages/beta/src/types.ts	44.11% <33.33%> (-3.89%)	⬇️
packages/stable/src/api/3d/revisions3DApi.ts	95.65% <33.33%> (-4.35%)	⬇️
packages/core/src/baseCogniteClient.ts	74.54% <40.35%> (-17.46%)	⬇️
...eta/src/api/templateGroups/templateInstancesApi.ts	82.60% <50.00%> (-1.71%)	⬇️
packages/beta/src/cogniteClient.ts	96.96% <66.66%> (+0.09%)	⬆️
... and 83 more

[tommythorsen]

Does this change cause all header names to be lowercased, regardless of what case the users provide them in? If so, I'm not sure it's a good idea.

The ideal implementation would be case-insensitive, but case-preserving at the same time. Meaning that comparisons would be case-insensitive, but the original casing that the user provided the header names in, would be preserved.

[andersfylling]

I do see that debugging may be more difficult if the user input is transformed. I updated the code 👍