fixed VPack serializing of array with null values (issue #88)

Author: Mark

src/main/java/com/arangodb/velocypack/VPack.java

@@ -623,6 +623,8 @@ private void serializeArray(
 			final Object element = Array.get(value, i);
 			if (element != null) {
 				addValue(null, element.getClass(), element, builder, null, additionalFields);
+			} else {
+				builder.add(ValueType.NULL);
 			}
 		}
 		builder.close();

src/test/java/com/arangodb/velocypack/VPackSerializeDeserializeTest.java

@@ -692,9 +692,10 @@ public void fromArrayWithNull() {
 
 		final VPackSlice a1 = vpack.get("a1");
 		assertThat(a1.isArray(), is(true));
-		assertThat(a1.size(), is(1));
+		assertThat(a1.size(), is(2));
 		assertThat(a1.get(0).isString(), is(true));
 		assertThat(a1.get(0).getAsString(), is("foo"));
+		assertThat(a1.get(1).isNull(), is(true));
 	}
 
 	protected enum TestEnum {