Add MessageUnpacker.tryUnpackNil() method

When a code deals with an Optional<T> value, a common way is to
serialize a Nil value if the value is absent. To deserialize it, we
check whether the next value is nil or not first. If it is nil, skip the
byte. Otherwise, read a value. For this common use case, tryUnpackNil
simplifies the deserialization code. It does "check whether the next
value is nil or not first. If it is nil, skip the byte" in one method
call.

msgpack-core/src/main/java/org/msgpack/core/MessageUnpacker.java

@@ -389,7 +389,7 @@ private boolean ensureBuffer()
     public MessageFormat getNextFormat()
             throws IOException
     {
-        // makes sure that buffer has at leat 1 byte
+        // makes sure that buffer has at least 1 byte
         if (!ensureBuffer()) {
             throw new MessageInsufficientBufferException();
         }
@@ -731,6 +731,31 @@ public void unpackNil()
         throw unexpected("Nil", b);
     }
 
+    /**
+     * Peeks a Nil byte and reads it if next byte is a nil value.
+     *
+     * The difference from {@link unpackNil} is that unpackNil throws an exception if the next byte is not nil value
+     * while this tryUnpackNil method returns false without changing position.
+     *
+     * @return true if a nil value is read
+     * @throws MessageInsufficientBufferException when the end of file reached
+     * @throws IOException when underlying input throws IOException
+     */
+    public boolean tryUnpackNil()
+            throws IOException
+    {
+        // makes sure that buffer has at least 1 byte
+        if (!ensureBuffer()) {
+            throw new MessageInsufficientBufferException();
+        }
+        byte b = buffer.getByte(position);
+        if (b == Code.NIL) {
+            readByte();
+            return true;
+        }
+        return false;
+    }
+
     /**
      * Reads true or false.
      *

msgpack-core/src/test/scala/org/msgpack/core/MessagePackTest.scala

@@ -240,6 +240,19 @@ class MessagePackTest extends MessagePackSpec {
       check(null, _.packNil, { unpacker => unpacker.unpackNil(); null })
     }
 
+    "skipping a nil value" taggedAs ("try") in {
+      check(true, _.packNil, _.tryUnpackNil)
+      check(false, { packer => packer.packString("val") }, { unpacker => unpacker.tryUnpackNil() })
+      check("val", { packer => packer.packString("val") }, { unpacker => unpacker.tryUnpackNil(); unpacker.unpackString() })
+      check("val", { packer => packer.packNil(); packer.packString("val") }, { unpacker => unpacker.tryUnpackNil(); unpacker.unpackString() })
+      try {
+        checkException(null, { _ => }, _.tryUnpackNil)
+      }
+      catch {
+        case e: MessageInsufficientBufferException => // OK
+      }
+    }
+
     "pack/unpack integer values" taggedAs ("int") in {
       val sampleData = Seq[Long](Int.MinValue.toLong -
         10, -65535, -8191, -1024, -255, -127, -63, -31, -15, -7, -3, -1, 0, 2, 4, 8, 16, 32, 64, 128, 256, 1024, 8192, 65536,