Package ctxutil contains golang.org/x/net/context related utilities.
HTTPClient is the context key to use with golang.org/x/net/context's WithValue function to associate an *http.Client value with a context.
We use the same value as the oauth2 package (which first introduced this key) rather than creating a new one and forcing users to possibly set two.
Client returns the HTTP client to use for the provided context. If ctx is non-nil and has an associated HTTP client, that client is returned. Otherwise, http.DefaultClient is returned.