Revert "Fix single character variable"

This reverts commit e670916.

Author: adimiz1

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

@@ -277,8 +277,9 @@ public static String mergeToSingleUnderscore(String s) {
      */
     public static boolean isVariable(String s) {
         if (s == null ||
-                s.length() < 2 ||
-                !s.startsWith("$")) {
+                s.length() < 3 ||
+                !s.startsWith("$") ||
+                !Character.isLetter(s.charAt(1))) {
             return false;
         }