runtime - Emit default values in marshalled JSON

In order to make APIs more restful, require the marshaler to emit
default values. By default, it omits empty types. This causes
unexpected REST behavior by omitting all false values or zero-valued
integers.

If you like to override its behavior back, use the WithMarshalerOption:

```go
gwmux := runtime.NewServeMux(runtime.WithMarshalerOption(runtime.MIMEWildcard, &runtime.JSONPb{OrigName: true, EmitDefaults: false}))
```

Closes grpc-ecosystem#233

runtime/marshaler_registry.go

@@ -13,7 +13,7 @@ var (
 	acceptHeader      = http.CanonicalHeaderKey("Accept")
 	contentTypeHeader = http.CanonicalHeaderKey("Content-Type")
 
-	defaultMarshaler = &JSONPb{OrigName: true}
+	defaultMarshaler = &JSONPb{OrigName: true, EmitDefaults: true}
 )
 
 // MarshalerForRequest returns the inbound/outbound marshalers for this request.