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kotlinlang

It would be great if I could pass in an existing `GsonBuilder` and not only configure it in the `gson` block:
```
install(ContentNegotiation) (
  gson {
    setDateFormat(DateFormat.LONG)
  }
}
```
like this:
```
install(ContentNegotiation) (
  gson(existingGsonBuilder)
}
```

If you go to definition of `gson` function (<http://github.com/ktorio/ktor/blob/master/ktor-features/ktor-gson/src/io/ktor/gson/GsonSupport.kt#L85|github.com/ktorio/ktor/blob/master/ktor-features/ktor-gson/src/io/ktor/gson/GsonSupport.kt#L85>), you can see that it is very easy to use `existingGsonBuilder`.

Basically, it is
```
register(ContentType.Application.Json, GsonConverter(existingGsonBuilder.create()))
```