Added Utf8StringTest.

Author: conor10

src/test/java/org/web3j/abi/datatypes/Utf8StringTest.java

@@ -0,0 +1,17 @@
+package org.web3j.abi.datatypes;
+
+
+import org.junit.Test;
+
+import static org.hamcrest.core.Is.is;
+import static org.junit.Assert.assertThat;
+
+
+public class Utf8StringTest {
+
+    @Test
+    public void testToString() {
+        assertThat(new Utf8String("").toString(), is(""));
+        assertThat(new Utf8String("string").toString(), is("string"));
+    }
+}