Remove ETag from proxied responses, and add CORS header.

Removing the ETag drastically improves performance when proxying/caching for WMS servers that add it and then take just as long to revalidate the ETag as to send a full response (I'm looking at you, Esri).

Adding `Access-Control-Allow-Origin` allows our proxy to be used in cross-origin requests, which means we can use the production proxy when testing on localhost, for example.