修复验证码位数的bug，优化代码

src/test/java/com/wf/captcha/CaptchaTest.java

@@ -0,0 +1,27 @@
+package com.wf.captcha;
+
+import org.junit.Test;
+
+import java.io.File;
+import java.io.FileOutputStream;
+
+/**
+ * 测试类
+ * Created by 王帆 on 2018-07-27 上午 10:08.
+ */
+public class CaptchaTest {
+
+    @Test
+    public void test() throws Exception {
+        SpecCaptcha specCaptcha = new SpecCaptcha(150, 40, 4);
+        System.out.println(specCaptcha.text());
+        specCaptcha.out(new FileOutputStream(new File("D:/a/aa.png")));
+    }
+
+    @Test
+    public void testGIf() throws Exception {
+        GifCaptcha specCaptcha = new GifCaptcha(150, 40, 4);
+        System.out.println(specCaptcha.text());
+        specCaptcha.out(new FileOutputStream(new File("D:/a/aa.gif")));
+    }
+}