Fixing issue of passing varargs and string to Sprintf. Closes stretch…

…r#619

mock/mock.go

@@ -495,7 +495,7 @@ func (m *Mock) AssertCalled(t TestingT, methodName string, arguments ...interfac
 		}
 		if len(calledWithArgs) == 0 {
 			return assert.Fail(t, "Should have called with given arguments",
-				fmt.Sprintf("Expected %q to have been called with:\n%v\nbut no actual calls happened", methodName, arguments))
+				fmt.Sprintf("Expected %q to have been called with:\n%v\nbut no actual calls happened", methodName, []interface{}(arguments)))
 		}
 		return assert.Fail(t, "Should have called with given arguments",
 			fmt.Sprintf("Expected %q to have been called with:\n%v\nbut actual calls were:\n        %v", methodName, arguments, strings.Join(calledWithArgs, "\n")))
@@ -513,7 +513,7 @@ func (m *Mock) AssertNotCalled(t TestingT, methodName string, arguments ...inter
 	defer m.mutex.Unlock()
 	if m.methodWasCalled(methodName, arguments) {
 		return assert.Fail(t, "Should not have called with given arguments",
-			fmt.Sprintf("Expected %q to not have been called with:\n%v\nbut actually it was.", methodName, arguments))
+			fmt.Sprintf("Expected %q to not have been called with:\n%v\nbut actually it was.", methodName, []interface{}(arguments)))
 	}
 	return true
 }