Copy response http headers into grpc headers

[fisco-unimatic]

Yesterday I raised a pull request to copy the http response headers into the grpc trailers. After thinking about it some more whilst trying and failing to get to sleep last night, I decided they should probably be copied into the grpc headers instead. So now I'm raising this PR, too. I'll leave someone who knows more about this than me to decide which is right.

(This is to fix issue 128.)

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