Make ClassLoaderValue compatible with PowerMock (#402)

(cherry picked from commit 981fdd9)

core/src/main/java/org/kohsuke/stapler/ClassLoaderValue.java

@@ -45,14 +45,15 @@ protected T computeValue(Class<?> type) {
     public interface X {}
 
     public final T get(ClassLoader cl) {
+        Class<?> x;
         try {
-            cl.loadClass(X.class.getName());
-            // OK, X is visible to cl, can use trick
-        } catch (ClassNotFoundException x) {
+            x = cl.loadClass(X.class.getName());
+            // OK, X is visible to cl, can use trick; note that x != X.class when using PowerMock
+        } catch (ClassNotFoundException e) {
             // fallback, no caching; could use WeakHashMap though typically values would strongly hold keys so both could leak
             return computeValue(cl);
         }
-        Class<?> type = Proxy.getProxyClass(cl, X.class);
+        Class<?> type = Proxy.getProxyClass(cl, x);
         assert type.getClassLoader() == cl;
         return storage.get(type);
     }