Fix body on request, convert to json only when needed

[juvs]

There's an assumption when doing a request and pass body into..

...
    if (body != null) {
      String contentTypeHeader;
      if (body is String) {
        bodyString = body;
        contentTypeHeader = "text/plain";
      } else if (body is Map || body is List) {
        bodyString = Common.toJson(body);

        if (bodyEncoding == RequestBodyEncoding.JSON) {
          contentTypeHeader = "application/json";
        } else if (bodyEncoding == RequestBodyEncoding.FormURLEncoded) {
          contentTypeHeader = "application/x-www-form-urlencoded";
        } else {
          throw Exception('unsupported bodyEncoding "$bodyEncoding"');
        }
      }

      if (contentTypeHeader != null && !Common.hasKeyIgnoreCase(headers, "content-type")) {
        headers["content-type"] = contentTypeHeader;
      }
    }
...

When the body is Map or List, always convert to json... but in some scenarios this is wrong, for instance, in a request like this:

POST /api/login HTTP/1.1
Host: foo.com
Content-Type: application/x-www-form-urlencoded
Accept: */*

username=someusername&password=somepassword

Data needs to pass like key/value map, so only convert to json when you establish content type to "application/json", in any other case just pass raw body to the request.

[juvs]

Opened a pull request #13
Sorry for the rearrange of the code... Visual Code is doing far to much...

[jossef]

@juvs
Will merge & publish #13 later today

Thanks for your contribution!