Handle methods with Void return types properly (#675)

The DAP part of LSP4J was being too strict and failing to parse
response objects for methods with Void return type. With
Java 17 (or some version since Java 11) turning on more strict
reflection rules, the "An illegal reflective access operation
has occurred" that used to be a warning is now an exception
like "Failed making constructor 'java.lang.Void#Void()' accessible"

This change makes the code work and compatible with how LSP4J
handles LSP Void return types.

Fixes #674

org.eclipse.lsp4j.jsonrpc.debug/src/main/java/org/eclipse/lsp4j/jsonrpc/debug/adapters/DebugMessageTypeAdapter.java

@@ -389,7 +389,7 @@ private Message createMessage(String messageType, int seq, int request_seq, Stri
 								if (typeAdapter != null)
 									body = typeAdapter.fromJsonTree(jsonElement);
 								else
-									body = gson.fromJson(jsonElement, returnType);
+									body = fromJson(jsonElement, returnType);
 							}
 						}
 					}

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

@@ -37,6 +37,7 @@
 
 import com.google.gson.JsonArray;
 import com.google.gson.JsonObject;
+import com.google.gson.JsonPrimitive;
 import com.google.gson.reflect.TypeToken;
 
 public class DebugMessageJsonHandlerTest {
@@ -848,4 +849,77 @@ public void testParseSyntaxErrorRequest() {
 			Assert.assertNotNull(messageIssue.getCause());
 		}
 	}
+
+	private Object voidResponse(String body) {
+		Map<String, JsonRpcMethod> supportedMethods = new LinkedHashMap<>();
+		supportedMethods.put("foo", JsonRpcMethod.request("foo", new TypeToken<Void>() {
+		}.getType(), new TypeToken<Void>() {
+		}.getType()));
+		DebugMessageJsonHandler handler = new DebugMessageJsonHandler(supportedMethods);
+		handler.setMethodProvider((id) -> "foo");
+
+		String bodyField = "";
+		if (body != null) {
+			bodyField = ",body:" + body;
+		}
+
+		Message message = handler.parseMessage("{\"seq\":10," //
+				+ "\"type\":\"response\"," //
+				+ "\"request_seq\":4," //
+				+ "\"command\":\"foo\"," //
+				+ "\"success\":true"//
+				+ bodyField //
+				+ "}\n");
+		return ((ResponseMessage) message).getResult();
+	}
+
+	@Test
+	public void testVoidResponse_noBody() {
+		Assert.assertNull(voidResponse(null));
+	}
+
+	@Test
+	public void testVoidResponse_null() {
+		Assert.assertNull(voidResponse("null"));
+
+	}
+
+	@Test
+	public void testVoidResponse_primitive() {
+		Object result = voidResponse("true");
+		JsonPrimitive jsonPrimitive = (JsonPrimitive) result;
+		Assert.assertTrue(jsonPrimitive.getAsBoolean());
+	}
+
+	@Test
+	public void testVoidResponse_emptyArray() {
+		Object result = voidResponse("[]");
+		JsonArray jsonArray = (JsonArray) result;
+		Assert.assertTrue(jsonArray.isEmpty());
+	}
+
+	@Test
+	public void testVoidResponse_array() {
+		Object result = voidResponse("[1,2,3]");
+		JsonArray jsonArray = (JsonArray) result;
+		Assert.assertEquals(1, jsonArray.get(0).getAsInt());
+		Assert.assertEquals(2, jsonArray.get(1).getAsInt());
+		Assert.assertEquals(3, jsonArray.get(2).getAsInt());
+	}
+
+	@Test
+	public void testVoidResponse_emptryObject() {
+		Object result = voidResponse("{}");
+		Assert.assertTrue(result instanceof JsonObject);
+		JsonObject jsonObject = (JsonObject) result;
+		Assert.assertTrue(jsonObject.entrySet().isEmpty());
+	}
+
+	@Test
+	public void testVoidResponse_object() {
+		Object result = voidResponse("{\"allThreadsContinued\":false}");
+		Assert.assertTrue(result instanceof JsonObject);
+		JsonObject jsonObject = (JsonObject) result;
+		Assert.assertFalse(jsonObject.getAsJsonPrimitive("allThreadsContinued").getAsBoolean());
+	}
 }