Verify binary header base64 encoding/decoding against grpc spec.

[yang-g]

The spec says:

Implementations MUST accept padded and un-padded values and should emit un-padded values.

There should be a unit test to verify that's the case.

[yang-g]

Something like what go did in: grpc/grpc-go#1209

[ejona86]

I'm pretty certain we're sending padding, because I don't see a call to omitPadding(). It seems like we're safe with decoding the padding though, since BaseEncoding has in its docs: "Decoding rejects characters in the wrong case, though padding is optional."

We should have unit tests for decoding with and without padding.