Optimize ValuesMapToMeta()
The more headers there is, the better the new code will perform compared to the old code.
Original code:
BenchmarkValuesMapToMeta-10 3360139 342.8 ns/op 560 B/op 7 allocs/op
New code:
BenchmarkValuesMapToMeta-10 3642054 319.1 ns/op 560 B/op 3 allocs/op
The initial motivator was to avoid allocating an empty map and just returning nil
when input is empty. This is a common case for HTTP trailers.
Edited by Mikhail Mazurskiy