Wrap class OAuthException with !class_exists

Same problem and solution as discussed on
https://drupal.org/node/1334528

library/OAuth1.php

@@ -3,8 +3,10 @@
 
 /* Generic exception class
  */
-class OAuthException extends Exception {
+if (!class_exists('OAuthException')) {
-  // pass
+	class OAuthException extends Exception {
+		// pass
+	}
 }
 
 class OAuthConsumer {