Add separate request and response format

Add `Request.RequestFormat`, `Request.ResponseFormat`, and `Request.Accept` for setting separate request format (e.g. form-urlencoded) and response format (e.g. json)

```VB
Dim Request As New RestRequest
Request.RequestFormat = formurlencoded
Request.ResponseFormat = json

Request.AddParameter "a", 123
Request.AddParameter "b", 456
Request.Method = httpPOST

' Execute -> Body: a=123&b=456
' Response -> Body: {"a":123,"b":456}
```

Fixes #45