Fix single character variable

Author: adimiz1

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

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

cloudinary-core/src/test/java/com/cloudinary/transformation/VariableTest.java

@@ -0,0 +1,42 @@
+package com.cloudinary.transformation;
+
+import com.cloudinary.Cloudinary;
+import com.cloudinary.Transformation;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TestName;
+
+import static org.junit.Assert.assertEquals;
+
+public class VariableTest {
+
+    private Cloudinary cloudinary;
+
+    @Rule
+    public TestName currentTest = new TestName();
+
+    @Before
+    public void setUp() {
+        System.out.println("Running " + this.getClass().getName() + "." + currentTest.getMethodName());
+        this.cloudinary = new Cloudinary("cloudinary://a:b@test123?load_strategies=false");
+    }
+
+    @Test
+    public void testSingleCharacterVariable() {
+        String url = cloudinary.url().transformation(new Transformation().variable("$a","100").chain().height("$a")).generate("sample.jpg");
+        assertEquals("http://res.cloudinary.com/test123/image/upload/$a_100/h_$a/sample.jpg", url);
+    }
+
+    @Test
+    public void testDollarOnlyAsVariable() {
+        String url = cloudinary.url().transformation(new Transformation().variable("$","100").chain().height("$")).generate("sample.jpg");
+        assertEquals("http://res.cloudinary.com/test123/image/upload/h_$/sample.jpg", url);
+    }
+
+    @Test
+    public void testMultipleCharactersVariable() {
+        String url = cloudinary.url().transformation(new Transformation().variable("$a45","100").chain().height("$a45")).generate("sample.jpg");
+        assertEquals("http://res.cloudinary.com/test123/image/upload/$a45_100/h_$a45/sample.jpg", url);
+    }
+}