Added a test with a NULL string

JsonGeneratorTests/StringBuilder.cpp

@@ -8,6 +8,11 @@
 
 void StringBuilder::append(const char* s)
 {
+    if (!s)
+    {
+        return append("null");        
+    }
+
     char* tail = buffer + length;
 
     while (*s && length<capacity)
@@ -20,7 +25,11 @@ void StringBuilder::append(const char* s)
 
 void StringBuilder::appendEscaped(const char* s)
 {
-    if (length > capacity - 3) return;
+    if (length > capacity - 2)
+    {
+        // not enough from for quotes
+        return;
+    }
 
     buffer[length++] = '"';
 

JsonGeneratorTests/StringBuilderAppendTests.cpp

@@ -28,6 +28,12 @@ namespace JsonGeneratorTests
             assertResultIs("");
         }
 
+        TEST_METHOD(Null)
+        {
+            append(NULL);
+            assertResultIs("null");
+        }
+
         TEST_METHOD(OneString)
         {
             append("ABCD");