strings.Join has two allocations:
(1) []byte intermediate
(2) final string conversion
The comma-separated keys are ultimately stored in hpack.HeaderField.Value
as a string so (2) is not wasteful. Because strings.Join is used so heavily
I assume its implementation is optimal - (1) is necessary.
Therefore, short of refactoring hpack.HeaderField's Value type, the status quo
seems optimal.

This TODO appears to have been copy-pasted from net/http/transfer.go
However, the two cases are different. transfer.go has two allocations
in addition to strings.Join: concatenation with literals and converting
string to bytes in io.WriteString.

Change-Id: I292203a6535dd2774f5bf45e7280b89aabb5d78d
Reviewed-on: https://go-review.googlesource.com/34626Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>