Change SuperAgent.Header type to http.Header for support multiple values

[edentsai]

1. Change SuperAgent.Header type to http.Header for supports multiple values.
2. Add AppendHeader() to support multiple values of Header, for example:

// Accept: application/json, text/plain
gorequest.New().
        AppendHeader("Accept", "application/json").
        AppendHeader("Accept", "text/plain").
        Type("text").
        Send("data").
        End()

3. Add some test cases for force Type().

[parnurzeal]

Thank you for this. Clean and sensible approach.
Also, sorry for the delay reviewing.