Just make ProviderOpts a map[string]string.

I have no idea why I made it a map[string]interface{} and it's a pain to deal
with.

server/auth/google/google.go

@@ -35,7 +35,7 @@ func New(c *config.Auth) auth.Provider {
 			Endpoint:     google.Endpoint,
 			Scopes:       []string{googleapi.UserinfoEmailScope, googleapi.UserinfoProfileScope},
 		},
-		domain: c.ProviderOpts["domain"].(string),
+		domain: c.ProviderOpts["domain"],
 	}
 }
 

server/config/config.go

@@ -24,7 +24,7 @@ type Auth struct {
 	OauthClientSecret string            `mapstructure:"oauth_client_secret"`
 	OauthCallbackURL  string            `mapstructure:"oauth_callback_url"`
 	Provider          string            `mapstructure:"provider"`
-	ProviderOpts      map[string]interface{} `mapstructure:"provider_opts"`
+	ProviderOpts      map[string]string `mapstructure:"provider_opts"`
 	JWTSigningKey     string            `mapstructure:"jwt_signing_key"`
 }