fix: case-sensitive header lookup in HttpResponse getValue method

[jm-riley]

Description

Currently, the getValue method in HttpResponse performs a case-sensitive lookup of header keys after lowercasing the input key. This causes issues when response headers like "Digest" need to be retrieved as part of verification.

While creating the transient token, response from /flex/v2/tokens request contains a response header of Digest. Calling DigestHelper.verifyResponseDigest() will always throw a missingDigestHeader error when attempting to assign digestHeader value:

guard let digestHeader = response.getValue(for: "Digest") else {
    throw FlexInternalErrors.missingDigestHeader.errorResponse
}

This change modifies the HttpResponse.getValue() function to:

1. First attempt an exact key match
2. Only apply case-insensitive comparison if exact match fails