Drop cache headers from error responses.

Closes #107 (github), JIRA CDB-1423 #resolve

NOTE: does not choke on headers cleanup when response headers are not set,
      but raises a WARNING instead (JIRA CDB-1438 #resolve)

lib/cartodb/cartodb_windshaft.js

@@ -34,6 +34,20 @@ var CartodbWindshaft = function(serverOptions) {
       return version;
     }
 
+    // Override sendError to drop added cache headers (if any)
+    // See http://github.com/CartoDB/Windshaft-cartodb/issues/107
+    var ws_sendError = ws.sendError;
+    ws.sendError = function(res) {
+      if ( res._headers ) {
+        delete res._headers['cache-control'];
+        delete res._headers['last-modified'];
+        delete res._headers['x-cache-channel'];
+      } else {
+        console.log("WARNING: response has no _headers: "); console.dir(res);
+      }
+      ws_sendError.apply(this, arguments);
+    };
+
     /**
      * Helper to allow access to the layer to be used in the maps infowindow popup.
      */