Add alternative for headers and query for RpcHttp that don't rely on "map"

[mhoeger]

Need to make this change to flow headers and query even if the values are empty/null
Root cause: #21

[mhoeger]

I was OOF this sprint and didn't get a chance to look into this - bumping to next sprint.
This change is required to reduce the amount of duplicated data passed over the wire per invocation.
(cc: @mathewc)