Metadata with non-ASCII characters should panic

[ishitatsuyuki]

What version of gRPC are you using?

master (no vendoring)
libprotoc 3.2.0

What version of Go are you using (go version)?

go version go1.9.2 windows/amd64

What did you do?

	header := metadata.Pairs("x-header-key", "ｕｎｉｃｏｄｅ")
	trailer := metadata.Pairs("x-trailer-key", "ｕｎｉｃｏｄｅ")
	grpc.SendHeader(ctx, header)
	grpc.SetTrailer(ctx, trailer)

What did you expect to see?

This should panic because it violates the HTTP protocol.

What did you see instead?

This didn't panic and sent non compliant response out.

[dfawley]

Panicking should be avoided if at all possible. SetHeader and SendHeader could return errors. Unfortunately, grpc's ServerStream.SetTrailer doesn't return an error. In this case, one other option would be to make the transport force the stream to result in an error with an error message indicating that the server attempted to send malformed metadata.

[menghanl]

Duplicate of #468