Simplify the check for object keys that are themselves objects.

For object keys, we can just skip the part of `nextValue()` that parses values
that are objects or arrays. Then the existing logic for unquoted values will
already stop at `{` or `[`, and that will produce a `Missing value` exception.

src/main/java/org/json/JSONTokener.java

@@ -428,10 +428,6 @@ public class JSONTokener {
         case '"':
         case '\'':
             return this.nextString(c);
-        case '{':
-            throw syntaxError("Nested object not expected here.");
-        case '[':
-            throw syntaxError("Nested array not expected here.");
         }
 
         /*

src/test/java/org/json/junit/JSONObjectTest.java

@@ -2230,7 +2230,7 @@ public class JSONObjectTest {
             assertNull("Expected an exception",new JSONObject(str));
         } catch (JSONException e) {
             assertEquals("Expecting an exception message",
-                "Nested object not expected here. at 2 [character 3 line 1]",
+                "Missing value at 1 [character 2 line 1]",
                 e.getMessage());
         }
         try {
@@ -2239,7 +2239,7 @@ public class JSONObjectTest {
             assertNull("Expected an exception",new JSONObject(str));
         } catch (JSONException e) {
             assertEquals("Expecting an exception message",
-                "Nested array not expected here. at 10 [character 11 line 1]",
+                "Missing value at 9 [character 10 line 1]",
                 e.getMessage());
         }
         try {