/__reload route does not use custom headers

When starting `apisprout --header "CustomerHeader: value" uri`, apisprout will as expected load the swagger definition at uri provided using the custom header provided. We use it to provide a security access token to access the protected uri resource.

When asking apisprout to reload the definition via a /__reload call:
* custom header won't be used
* download will fail (our server reports a 404 if our security heaser isn't sent)
* apisprout will display 'Reloaded from uri" message and no error is reported
* apisprout won't fail and will keep on working assuming an empty definition has been provided (all calls to apisprout will result in 404)

Will try to provide a fix via a PR.

Start by tracing the /__reload route and compare it with the initial apisprout --header definition-loading path. Verify that configured custom headers are sent during reload and that download failures are reported instead of producing an empty definition; done means protected definitions reload successfully and errors are visible.

Written by the indexing model from the issue text.