internal/transport: fix a bug causing -bin metadata to be incorrectly encoded #3985
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Most interfaces for adding metadata to a context downcase keys, except metadata.AppendToOutgoingContext (presumably for performance reasons). Documentation for metadata.AppendToOutgoingContext, referring to that of metadata.Pairs, states that metadata keys are downcased.
The bug is manifest if you use metadata.AppendToOutgoingContext to add metadata with a non-downcased
-Bin
suffix. http2Client will not encode such metadata, as encodeMetadataHeader just performs a lower suffix check.