Add support for single character variable

Author: adimiz1

cloudinary-core/src/main/java/com/cloudinary/utils/StringUtils.java

@@ -277,7 +277,7 @@ public static String mergeToSingleUnderscore(String s) {
      */
     public static boolean isVariable(String s) {
         if (s == null ||
-                s.length() < 3 ||
+                s.length() < 2 ||
                 !s.startsWith("$") ||
                 !Character.isLetter(s.charAt(1))) {
             return false;

cloudinary-core/src/test/java/com/cloudinary/UtilTest.java

@@ -76,8 +76,8 @@ public void testIsVariable(){
         assertTrue(StringUtils.isVariable("$ab"));
         assertTrue(StringUtils.isVariable("$asdasda"));
         assertTrue(StringUtils.isVariable("$a34asd12e"));
+        assertTrue(StringUtils.isVariable("$a"));
 
-        assertFalse(StringUtils.isVariable("$a"));
         assertFalse(StringUtils.isVariable("sda"));
         assertFalse(StringUtils.isVariable("   "));
         assertFalse(StringUtils.isVariable("... . /"));
@@ -159,4 +159,4 @@ public void testRemoveStartingChars(){
         assertEquals("bcde", StringUtils.removeStartingChars("aaaaaabcde", 'a'));
         assertEquals("bcdeaa", StringUtils.removeStartingChars("aaaaaabcdeaa", 'a'));
     }
-}
+}