Fix: Content-Disposition header back in makeRequest handler via ProxyBase

[johnfargo]

Reviewers: shindig.remailer_gmail.com,


Please review this at http://codereview.appspot.com/100050

Affected files:
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyBase.java
   
java/gadgets/src/main/java/org/apache/shindig/gadgets/servlet/ProxyHandler.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/ProxyBaseTest.java
   
java/gadgets/src/test/java/org/apache/shindig/gadgets/servlet/ProxyHandlerTest.java