Update test to ignore OS specific flag

Due to internal changes of the JDK the OS flag value of the
gzip header changed. This caused an unexpected base64
to be generated. This fix ignores the OS specific flag when
verifying the signature.

DEVSIX-7177

Author: guustysebie

commons/src/test/java/com/itextpdf/commons/utils/Base64Test.java

@@ -39,12 +39,30 @@ public void testEncodeObjectForNullObject() {
 
     @Test
     public void testDecodeForSourceIsEmptyArray() {
-        Assert.assertEquals("", Base64.encodeBytes(new byte[]{}));
+        Assert.assertEquals("", Base64.encodeBytes(new byte[] {}));
     }
 
     @Test
     public void testDecodeForSourceIsEmptyArrayGzip() {
-        Assert.assertEquals("H4sIAAAAAAAAAAMAAAAAAAAAAAA=", Base64.encodeBytes(new byte[]{}, Base64.GZIP));
+        //HEADER        FOOTER          example of signatures that can be generated
+        //H4sIAAAAAAAAAAMAAAAAAAAAAAA=	1f 8b 08 00 00 00 00 00 00 00 03 00 00 00 00 00 00 00 00 00 // windows
+        //H4sIAAAAAAAA/wMAAAAAAAAAAAA=  1f 8b 08 00 00 00 00 00 00 ff 03 00 00 00 00 00 00 00 00 00 // unknown
+        //H4sIAAAAAAAAAwMAAAAAAAAAAAA=	1f 8b 08 00 00 00 00 00 00 03 03 00 00 00 00 00 00 00 00 00 // linux
+        //H4sIAAAAAAAAAgMAAAAAAAAAAAA=	1f 8b 08 00 00 00 00 00 00 02 03 00 00 00 00 00 00 00 00 00 // vms
+        final String expectedHeaderWithoutOsFlag = "H4sIAAAAAAAA";
+        final String expectedFooter = "MAAAAAAAAAAAA=";
+        final int startIndexHeader = 0;
+        final int endIndexHeaderWithoutOsFlag = 12;
+        final int startIndexFooter = 14;
+        final int endIndexFooter = 28;
+
+        String generatedBase64 = Base64.encodeBytes(new byte[] {}, Base64.GZIP);
+
+        String generatedHeader = generatedBase64.substring(startIndexHeader, endIndexHeaderWithoutOsFlag);
+        String generatedFooter = generatedBase64.substring(startIndexFooter, endIndexFooter);
+
+        Assert.assertEquals(expectedHeaderWithoutOsFlag, generatedHeader);
+        Assert.assertEquals(expectedFooter, generatedFooter);
     }
 
 }