func ContextForClient(h *http.Client) context.Context {
	if h == nil {
		return oauth2.NoContext
	}
	return context.WithValue(oauth2.NoContext, oauth2.HTTPClient, h)
}
ContextForClient provides a context for use with oauth2.
package goth
import (
	"context"
	"fmt"
	"net/http"
	"golang.org/x/oauth2"
)
type Provider interface {
	Name() string
	SetName(name string)
	BeginAuth(state string) (Session, error)
	UnmarshalSession(string) (Session, error)
	FetchUser(Session) (User, error)
	Debug(bool)
	RefreshToken(refreshToken string) (*oauth2.Token, error)  
	RefreshTokenAvailable() bool                              
}
const NoAuthUrlErrorMessage = "an AuthURL has not been set"
type Providers map[string]Provider
var providers = Providers{}
func UseProviders(viders ...Provider) {
	for _, provider := range viders {
		providers[provider.Name()] = provider
	}
}
func GetProviders() Providers {
	return providers
}
func GetProvider(name string) (Provider, error) {
	provider := providers[name]
	if provider == nil {
		return nil, fmt.Errorf("no provider for %s exists", name)
	}
	return provider, nil
}
func ClearProviders() {
	providers = Providers{}
}