[fix] Fix handling for Streams in IConnection for raw content

[martincostello]

Fix NullReferenceException if raw content was handled as a string rather than a stream by HttpClientAdapter.BuildResponse().

I haven't added a test for this, but have validated that the apps that surfaced this issue are working when using a local build of Octokit including this change. If you can direct me to exactly where you'd like a test for this, I'll add one.

Resolves #2789

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Before the change?

• A NullReferenceException would be thrown.

After the change?

• The call succeeds.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

• Yes
• No

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[martincostello]

Looks like this got closed by accident because I put the wrong # in the commit message for the other PR, and I can't re-open it.

[martincostello]

@nickfloyd Anything more you need from me to progress this fix?

[kfcampbell]

I do think it'd be possible to write a test for this case in Octokit.Tests/Http/ConnectionTests.cs, since the modified GetRaw is called by two public functions. Why don't we try doing so?

[kfcampbell]

Looks good, thank you!