cookie: add possibility to disable same site attribute

[wja513]

fix some old browsers cann't set the cookie if it contains "SameSite" attribute.

ctx.Cookie(c, &fiber.Cookie{
	Name:     cookie.Name,
	Value:    cookie.Value,
        ...
	//SameSite: "",// this attribute should be constant.（why not use fasthttp constant directly?）
})

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[ReneWerner87]

@wja513 please check the tests

please add new ones for your customization

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