Incorrect mapping from grpc code to http code for canceled request.

[brijeshvishwanath]

Fixes #2707

Brief description of what is fixed or changed

Incorrect mapping from grpc code to http code for canceled request.

The reference for error mapping in HTTPStatusFromCode is followed from: https://github.com/googleapis/googleapis/blob/942691f8dcf3e521be35d909de9bba3239feb471/google/rpc/code.proto#L40 And the reference claims that http code mapping for CANCELED grpc code is 499 (Client Closed Request), and not 408 (StatusRequestTimeout).

This MR changes this mapping.

Note: HTTP error code 499 const is not supported by go http pkg, hence I have used the integer literal.

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[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[johanbrandhorst]

Ping, could you please sign the CLA to have this merged?

[johanbrandhorst]

This has been contributed by someone else.