Generate Robust C# Code

## Improvement Suggestion:
I would have liked the following autogenerated C# for a HTTP POST request with the single parameter `err` of value `+ &` serialized as _x-www-form-urlencoded_ in the body.
```
var client = new RestClient("https://localhost");
var request = new RestRequest(Method.POST);
request.AddHeader("postman-token", "da4d0044-61a9-3943-40db-9e753aca6020");
request.AddHeader("cache-control", "no-cache");
Action parameter = (name, value) => request.AddParameter(name, value);
parameter("err", "+ &");
IRestResponse response = client.Execute(request);
```

but what Postman did produce for me was the following C# code:

```
var client = new RestClient("https://localhost");
var request = new RestRequest(Method.POST);
request.AddHeader("postman-token", "da4d0044-61a9-3943-40db-9e753aca6020");
request.AddHeader("cache-control", "no-cache");
request.AddHeader("content-type", "application/x-www-form-urlencoded");
request.AddParameter("application/x-www-form-urlencoded", "err=%2B%20%26", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
```

## Rationale
1. `parameter("err", "+ &");` is more readable than "[..] `err=%2B%20%26` [..]". The value, `+ &`, is more recognizable.
2. The code is more maintanable. At work, I tried the following development workflow:
(a) Type a minimal working example request into to Postman.
(b) Generate C# code.
(c) Copy generated code to Visual Studio.
(d) Replace example values by C# method parameters.
This workflow failed, because the method parameters were not _x-www-form-urlencoded_ at runtime. Generating UrlEncode() calls would make writing a consumer of a HTTP POST _x-www-form-urlencoded_ API effortless.
3. This usage of `IRestRequest.AddParameter` is encouraged, whereas the currently generated code depends on an internal hack not meant for 3rd party usage.
```/// - RequestBody: Used by AddBody() (not recommended to use directly)```

## App Details:
```
Postman for Windows
Version 5.3.2
win32 6.1.7601 / x64
```

Continuation of: https://github.com/postmanlabs/postman-app-support/issues/3870

Start at the C# snippet-generation entry point and compare the current form-urlencoded output with the issue’s two examples, focusing on how parameter names and values are represented. Done means generated C# uses request parameters with runtime-safe URL encoding rather than embedding one encoded request-body string.

索引モデルが issue の本文から書いたものです。