added number scopes. added test case coverage

Author: S-M-J-I

src/main/java/com/thealgorithms/stacks/PalindromeWithStack.java

@@ -37,12 +37,10 @@ public boolean checkPalindrome(String string) {
         // Iterate through the string
         for (int i = 0; i < lowercase.length(); ++i) {
             char c = lowercase.charAt(i);
-            if (c >= 'a' && c <= 'z') {
-                // Build the string from L->R
-                stringBuilder.append(c);
-                // Push to the stack
-                stack.push(c);
-            }
+            // Build the string from L->R
+            stringBuilder.append(c);
+            // Push to the stack
+            stack.push(c);
         }
 
         // The stack contains the reverse order of the string

src/test/java/com/thealgorithms/stacks/PalindromeWithStackTest.java

@@ -59,7 +59,19 @@ public void testBlankString() {
 
     @Test
     public void testStringWithNumbers() {
-        String testString = "bio123ib";
+        String testString = "12321";
+        assertTrue(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testStringWithNumbersTwo() {
+        String testString = "12325";
+        assertFalse(palindromeChecker.checkPalindrome(testString));
+    }
+
+    @Test
+    public void testStringWithNumbersAndLetters() {
+        String testString = "po454op";
         assertTrue(palindromeChecker.checkPalindrome(testString));
     }
 }