Authenticated HTTP Request Fails Only in FSI

I make an authenticated request as follows:

let url = "https://datafeeds.networkrail.co.uk/ntrod/CifFileAuthenticate?type=CIF_ALL_FULL_DAILY&day=toc-full"
let auth = "Basic " + (username + ":" + password |> Encoding.UTF8.GetBytes |> Convert.ToBase64String)
Http.Request(url, headers=["Authorization", auth])

Http.Request - FSharp.Data

It works fine in a compiled F # application, but when I run this on F # interactive, the sent request is a little different, and I get this error:

System.Net.WebException: The remote server returned an error: (403) Forbidden.
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message>

Looking at the violinist, this url returns 302, which redirects to https://nr-datafeed-cif.s3.amazonaws.com/CIF_ALL_FULL_DAILY%2Ftoc-full.json ? ...

But according to fsi, subsequent requests are sent https://nr-datafeed-cif.s3.amazonaws.com/CIF_ALL_FULL_DAILY/toc-full.json ? ....