Default uri expansion all use ExpandWithDefault now.
In order to keep everything consistent, parameters for uri expansion are named uriParams while parameters sent with the request are named requestParams.
Return type changed to pointers to be consistent with the rest of go-octokit.
Failure cases changed to default to nil rather than returning an empty struct or an empty slice.
ExpandWithDefaultnow.uriParamswhile parameters sent with the request are namedrequestParams.nilrather than returning an empty struct or an empty slice.