Mark GinI18n recievers as public

[chivincent]

Why?

For testing, users could mock GinI18n implementation, but they cannot do.

c, _ := gin.CreateTestContext(httptest.NewRecorder())
c.Set("i18n", &mockedGinI18N{})

if i18n.MustGetMessage(c, "hello_world") != "hello world" {
    t.Fatal()
}

However, the GinI18n interface defines 4 receivers:

getMessage(context *gin.Context, param interface{}) (string, error)
mustGetMessage(context *gin.Context, param interface{}) string
setBundle(cfg *BundleCfg)
setGetLngHandler(handler GetLngHandler)

Users cannot define mockedGinI18N in their code, because GinI18n has non-exported method.

How?

In this PR, I fixed that GinI18n interface receivers with public.