fix nnbd http parsing bug for http_headers_test
fix nnbd http parsing bug for http_headers_test

This fixs part of http_headers_test. The rest is because HeaderValue becomes non-nullable(in this cl https://dart-review.googlesource.com/c/sdk/+/134722).

Change-Id: Id1aad86363564986dba5282a9983924252ae0ecb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135530
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Zichang Guo <d26af3001798f91912fb1504666bca93c093d553@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk