Add Checkstyle EqualsHashCode

Author: garydgregory

src/conf/checkstyle.xml

@@ -41,6 +41,7 @@ limitations under the License.
     <module name="EmptyBlock">
       <property name="option" value="text" />
     </module>
+    <module name="EqualsHashCode" />
     <module name="FinalLocalVariable" />
     <module name="ImportOrder">
       <property name="option" value="top"/>

src/test/java/org/apache/commons/io/function/IOPredicateTest.java

@@ -50,6 +50,11 @@ public class IOPredicateTest {
         public boolean equals(final Object obj) {
             throw Erase.rethrow(new IOException("Expected"));
         }
+        @Override
+        public int hashCode() {
+            // Pair implementation with equals() even though not strictly necessary.
+            return super.hashCode();
+        }
     };
 
     private static final Predicate<Object> THROWING_UNCHECKED_PREDICATE = TestConstants.THROWING_IO_PREDICATE.asPredicate();