My problem lies in the fact that we currently support this case and use it quite a bit internally. The existing implementation uses form encoding.

```java
@POST("/foo")
Response thing(@Named("first") String first, @Named("last") String last);
```
```java
Response r = service.thing("Jesse", "Wilson");
```
...becomes...
```
POST /foo HTTP/1.1
Content-Type: application/x-www-form-urlencoded

first=Jesse&last=Wilson
```

I can make `Converter` an abstract class and default to this behavior, only overriding in our default GSON converter?