Uses `HTTP_X_HUB_STORE` instead of `HTTP_X_HUB-STORE`. Although we pass
`curl -H 'X-HUB-TOKEN:...' ...` Rack always converts the case, dashes and
add the HTTP_ prefix.

http://rack.rubyforge.org/doc/SPEC.html
https://tools.ietf.org/html/rfc3875#section-4.1.18