Specify charset returned from OpenSearch.

[dblock]

Description

Coming from #286 where I started running dredd against the spec. It fails with the following.

fail: headers: At '/content-type' No enum match for: "application/json; charset=utf-8"

expected: 
headers: 
    Content-Type: application/json

actual: 
statusCode: 200
headers: 
    content-type: application/json; charset=UTF-8

In this PR we match the charset correctly in the spec.

I think this is right, but I am not sure.

Issues Resolved

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[github-actions]

API specs implemented for 266/649 (40%) APIs.

[github-actions]

API specs implemented for 266/649 (40%) APIs.

[Xtansia]

Was wondering why this wasn't an issue with the previous dredd setup, turns out we just explicitly changed the expectation in the hooks files

opensearch-api-specification/test/models/cat/indices/hooks.js

Line 24 in c256d99

transactions.expected.headers['Content-Type'] = "application/json; charset=UTF-8";

[dblock]

Was wondering why this wasn't an issue with the previous dredd setup, turns out we just explicitly changed the expectation in the hooks files

opensearch-api-specification/test/models/cat/indices/hooks.js

Line 24 in c256d99

transactions.expected.headers['Content-Type'] = "application/json; charset=UTF-8";

Interesting! I couldn't find any explicit needs to specify the charset in the spec, so I am going to do the above hack.