#93 fix control character serialization

Author: taowen

src/main/java/com/jsoniter/output/StreamImplString.java

@@ -132,7 +132,7 @@ private static void writeStringSlowPath(JsonStream stream, String val, int i, in
         if (escapeUnicode) {
             for (; i < valLen; i++) {
                 int c = val.charAt(i);
-                if (c > 125) {
+                if (c > 125 || c < 32) {
                     byte b4 = (byte) (c & 0xf);
                     byte b3 = (byte) (c >> 4 & 0xf);
                     byte b2 = (byte) (c >> 8 & 0xf);

src/test/java/com/jsoniter/output/TestString.java

@@ -8,4 +8,8 @@ public void test_unicode() {
         String output = JsonStream.serialize(new Config.Builder().escapeUnicode(false).build(), "中文");
         assertEquals("\"中文\"", output);
     }
+    public void test_escape_control_character() {
+        String output = JsonStream.serialize(new String(new byte[]{0}));
+        assertEquals("\"\\u0000\"", output);
+    }
 }