Add function to delete internal context.data

[byanjiong]

We use c.Set("isUser", true) to pass data to other handlers, sometimes in complex functions, we may use a few handlers inside a handler, some handlers may required some internal data to be set while other handlers do not need those data, so hope to have a function to "delete" the context.data

func abc123(c *routing.Context) error {

    c.Set("name1", true)
    err := cde123(c)

    c.Delete("name1") // <-- hope to have something like that to delete internal data
    err := cde123(c)

    return nil
}