[Amazon.Lambda.RuntimeSupport] fix: Find header key with insensitive comparison

[duncanista]

Issue

Idea for #2093

Changes

Looks up for headers without any case restrictions.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[normj]

The change looks good but can you rebase the PR to the dev branch? The PR has picked up some commits that are in master that haven't yet been backported back to dev.

Also the PR needs a change file to take care of change log and versioning. Can you add the following content, feel free to change the change log messages, in the following file: .autover\changes\<random-guid>.json

{
  "Projects": [
    {
      "Name": "Amazon.Lambda.RuntimeSupport",
      "Type": "Patch",
      "ChangelogMessages": [
        "Fix issue making HTTP header comparisons be case insensitive"
      ]
    }
  ]
}