add testConstructorWithNullObject case in ReflectionToStringBuilderTest.java

src/test/java/org/apache/commons/lang3/builder/ReflectionToStringBuilderTest.java

@@ -0,0 +1,12 @@
+package org.apache.commons.lang3.builder;
+
+import org.junit.Test;
+
+public class ReflectionToStringBuilderTest {
+
+    @Test(expected=IllegalArgumentException.class)
+    public void testConstructorWithNullObject() {
+        new ReflectionToStringBuilder(null, ToStringStyle.DEFAULT_STYLE, new StringBuffer());
+    }
+
+}