Add headers to allow for caching API backends that set HTTP basic auth.

This is part of 18F/api.data.gov#281

The bulk of this will be implemented (with tests) over in the router
project, since that's where the caching stuff lives. But this allows us
to distinguish between authorization headers we add due to api backend
config versus externally sent authorization headers.

lib/gatekeeper/middleware/rewrite_request.js

@@ -86,7 +86,7 @@ _.extend(RewriteRequest.prototype, {
     }
 
     // Never pass along basic auth if it's how the api key was passed in
-    // (otherwise, we don't want to touch the basica auth and pass along
+    // (otherwise, we don't want to touch the basic auth and pass along
     // whatever it contains)..
     if(request.apiUmbrellaGatekeeper.user && request.basicAuthUsername === request.apiUmbrellaGatekeeper.apiKey) {
       delete request.headers.authorization;
@@ -147,6 +147,9 @@ _.extend(RewriteRequest.prototype, {
     if(auth) {
       var base64 = (new Buffer(auth, 'ascii')).toString('base64');
       request.headers.Authorization = 'Basic ' + base64;
+      request.headers['X-Api-Umbrella-Allow-Authorization-Caching'] = 'true';
+    } else {
+      delete request.headers['x-api-umbrella-allow-authorization-caching'];
     }
   },