Add missing quotes around body in JSON

Gson was parsing this malformed JSON, but it wasn't actually correct
without this escaping.

Before the message would look like:

```json
{"seq":10,"type":"response","request_seq":4,"command":"foo","success":true,body:{"allThreadsContinued":false}}
```

Now it looks like:

```json
{"seq":10,"type":"response","request_seq":4,"command":"foo","success":true,"body":{"allThreadsContinued":false}}

```

org.eclipse.lsp4j.jsonrpc.debug/src/test/java/org/eclipse/lsp4j/jsonrpc/debug/test/json/DebugMessageJsonHandlerTest.java

@@ -860,7 +860,7 @@ private Object voidResponse(String body) {
 
 		String bodyField = "";
 		if (body != null) {
-			bodyField = ",body:" + body;
+			bodyField = ",\"body\":" + body;
 		}
 
 		Message message = handler.parseMessage("{\"seq\":10," //