Added test to ensure correct behaviour for derived optionals (YousefED#126)

Author: MarcusRiemer

Diff for: test/programs/optionals-derived/main.ts

@@ -0,0 +1,9 @@
+interface MyBase {
+    baseRequired : number;
+    baseOptional?: number;
+}
+
+interface MyDerived extends MyBase {
+    derivedRequired : number;
+    derivedOptional?: number;
+}

Diff for: test/programs/optionals-derived/schema.json

@@ -0,0 +1,19 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema#",
+    "type": "object",
+    "required": [ "baseRequired", "derivedRequired"  ],
+    "properties": {
+        "baseRequired": {
+            "type": "number"
+        },
+        "baseOptional": {
+            "type": "number"
+        },
+        "derivedRequired": {
+            "type": "number"
+        },
+        "derivedOptional": {
+            "type": "number"
+        }
+    }
+}

Diff for: test/schema.test.ts

@@ -104,6 +104,7 @@ describe("schema", () => {
     assertSchema("type-nullable", "main.ts", "MyObject");
 
     assertSchema("optionals", "main.ts", "MyObject");
+    assertSchema("optionals-derived", "main.ts", "MyDerived");
 
     assertSchema("comments", "main.ts", "MyObject");
     assertSchema("comments-override", "main.ts", "MyObject");