I got this to work by following the Custom HTTP section of the OAuth documentation:
https://godoc.org/golang.org/x/oauth2#example-Config--CustomHTTP
"Note that if a custom *http.Client is provided via the Context it is used only for token acquisition and is not used to configure the *http.Client returned from NewClient."
Anyway, the effect on my API call limit is clear and discussions around the OAuth2 library aren't for this forum.