Allow for multiple values per key for application/x-www-form-urlencoded

[wchill]

Similar to #15, but for function calls like FlurlClient.PostUrlEncodedAsync.

Basically I have a form that takes in multiple values for a single key; the resulting encoded form data is something like key[]=value1&key[]=value2&key[]=value3. This would work if I were doing a GET with query string, but as I need to POST the data I have to use a workaround like such:

var kvp = new List<KeyValuePair<string, string>>();
foreach (var v in values)
{
	kvp.Add(new KeyValuePair<string, string>("key[]", v.ToString()));
}

var webpage = await url
	.WithClient(_client)
	.PostAsync(new FormUrlEncodedContent(kvp))
	.ReceiveString();

This works just fine, but when I use HttpTest I'm not able to assert what the request body should be because it throws the following exception:

Flurl.Http.FlurlHttpException: RequestBody is only available when Request.Content derives from Flurl.Http.Content.CapturedStringContent

Ideally when POSTing a urlencoded form, we can take the same approach as in #15 and enumerate if a value is of IEnumerable type.

[tmenier]

I think this is a good enhancement, but unfortunately I'm clearing the deck for work on Flurl.Http 2.0. No dates set but this might sit for a while. In the mean time you can certainly use this in your own projects by registering a custom serializer. I will get this merged into the core eventually. Thanks!