Serve static files with normal CORS headers (#1438)
Serve static files with normal CORS headers (#1438)

Currenlyt normal CORS headers are only set when AMP CORS headers are
required. This breaks the amp-3d-gltf sample.

The fix is to always the normal CORS headers and add AMP CORS headers if
the source origin get parameter is set.

Fixes #1437
ampproject/amp-by-example,ampproject/amp-by-example,ampproject/amp-by-example,ampproject/amp-by-example