Support `application/x-www-form-urlencoded`.

[ghosind]

Add the support of form data (without binary data), and set the value of the Content-Type field in the request header to application/x-www-form-urlencoded.

• For url.Values object, just call the Encoded method to encode it.
• For struct and map, check the type of all fields, convert the values to the strings, and encode them like url.Values object.

Example code

data := SomeType{
  F1: "Value 1",
  F2: 2,
  // and other data...
}

Request(dest, RequestConfig{
  Body: data,
  ContentType: "form"
})
// f1=Value 1&f2=2 // ...