Fix content language initialization and update to use headers backing field

[GarrettBeatty]

no dev config since its already in #4074

Description

update the backing field for content language to be in the headers instead so that for TransferUtilityDownloadResponse we can easily just copy the headers only instead of the response.ContentLanguage field.

Also was a bug in #4074 where it was always setting the content language in one case because the if statement didnt have brackets. So this fixes that bug and also makes it so the headers is the backing field

Motivation and Context

Testing

1. unit tests pass
2. need to run dry run

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[GarrettBeatty]

the headers collection doesnt have a setter https://github.com/aws/aws-sdk-net/pull/4168/files#diff-7d76ec6d3c33088a35cba25bbc63006459be9754c861936319892b11e3072652L117

so i believe we should be fine here

[GarrettBeatty]

With Get-Only: Guaranteed Single Instance

When Headers is get-only, you're guaranteed that there is exactly one HeadersCollection instance throughout the lifetime of the GetObjectResponse object:

var response = new GetObjectResponse();

// First access - creates the HeadersCollection
response.ContentLanguage = "en-US";
// Internally: response.Headers.ContentLanguage = "en-US"
//            ↓ This calls the getter, which creates headersCollection once

// Later access - uses THE SAME HeadersCollection
response.Headers.ContentType = "text/html";
//       ↑ Returns the same headersCollection instance

// Even later
Console.WriteLine(response.ContentLanguage); // "en-US" ✓
// Still reading from the SAME headersCollection instance

[peterrsongg]

we are still marshalling response.ContentLanguage somewhere though right?