Issue #35 - printToString doesn't encode to bytes

For any AbstractCharBasedFormatter, printToString can print directly to
a String and not have to convert to bytes and back again. This avoids
call to ByteArrayOutputStream.toString() at ProtobufFormatter:91 which
uses JVM default encoding instead of ProtobufFormatter.defaultCharset.
(I discovered this issue when trying to create JSON with a broad range
of unicode characters on a machine where the default Charset did not
cover the full Unicode range.

Author: tonicsoft

src/main/java/com/googlecode/protobuf/format/AbstractCharBasedFormatter.java

@@ -46,6 +46,18 @@ public void print(UnknownFieldSet fields, OutputStream output, Charset cs)
 
 	abstract public void print(UnknownFieldSet fields, Appendable output) throws IOException;
 
+	@Override
+	public String printToString(Message message) {
+		StringBuilder output = new StringBuilder();
+		try {
+			print(message, output);
+		} catch (IOException e) {
+			throw new RuntimeException("Writing to a StringBuilder threw an IOException (should never happen).",
+					e);
+		}
+		return output.toString();
+	}
+
 	@Override
 	public void merge(InputStream input, Charset cs, 
 			ExtensionRegistry extensionRegistry, Builder builder) throws IOException {

src/test/java/com/googlecode/protobuf/format/JsonFormatTest.java

@@ -9,6 +9,7 @@
 import org.testng.annotations.Test;
 import org.testng.reporters.Files;
 import protobuf_unittest.UnittestProto;
+import protobuf_unittest.UnittestProto.OneString;
 import sun.nio.cs.StandardCharsets;
 
 import java.io.IOException;
@@ -86,4 +87,15 @@ public void testDeserializeNullFieldFromJson() throws Exception {
         System.out.println(actual);
         assertThat(actual, equalTo(UnittestProto.TestNullField.newBuilder().build()));
     }
+
+    @Test
+    public void testSerializeToStringDoesNotRequireAnyEncoding() {
+        String aChineseCharacter = "\u2F76";
+        Charset encodingThatDoesntSupportChineseCharacters = Charset.forName("ASCII");
+        JsonFormat jsonFomat = new JsonFormat();
+        jsonFomat.setDefaultCharset(encodingThatDoesntSupportChineseCharacters);
+        OneString message = OneString.newBuilder().setData(aChineseCharacter).build();
+
+        assertThat(jsonFomat.printToString(message), is("{\"data\": \""+aChineseCharacter + "\"}"));
+    }
 }