add internal/storetesting JSONCallParams

This makes the tests look prettier and also
allows us to specify the body content
type without further burdening the call sites.
gofmt
Merge pull request #55 from rogpeppe/027-assertJSONCall-params

add internal/storetesting JSONCallParams

This makes the tests look prettier and also
allows us to specify the body content
type without further burdening the call sites.