Proper Javadoc example / add test for StringUtils

- Proper example for the purpose of javadoc and how it could look like
- Fix containsAny to be null safe
- Add tests

src/test/java/fr/xephi/authme/util/StringUtilsTest.java

@@ -0,0 +1,46 @@
+package fr.xephi.authme.util;
+
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Test for {@link StringUtils}.
+ */
+public class StringUtilsTest {
+
+    @Test
+    public void shouldFindContainedItem() {
+        // given
+        String text = "This is a test of containsAny()";
+        String piece = "test";
+
+        // when
+        boolean result = StringUtils.containsAny(text, "some", "words", "that", "do not", "exist", piece);
+
+        // then
+        assertThat(result, equalTo(true));
+    }
+
+    @Test
+    public void shouldReturnFalseIfNoneFound() {
+        // given
+        String text = "This is a test string";
+
+        // when
+        boolean result = StringUtils.containsAny(text, "some", "other", "words", null);
+
+        // then
+        assertThat(result, equalTo(false));
+    }
+
+    @Test
+    public void shouldReturnFalseForNullString() {
+        // given/when
+        boolean result = StringUtils.containsAny(null, "some", "words", "to", "check");
+
+        // then
+        assertThat(result, equalTo(false));
+    }
+}