Clear host header for cross-domain redirection

[VoidMonk]

Found a minor issue: if a request redirects from hostname A to hostname B, then the host header from the previous request must be cleared, else it will return a HTTP error 404 (or similar) from the destination server of hostname B.

Example httpbin test URL: https://httpbin.org/redirect-to?url=http%3A%2F%2Fexample.com%2F (redirects from httpbin.org to example.com).

Possible fix: add delete opts.headers['host'] to clear previous request's host header in parseOptsUrl() around line 103.

As a sidenote, you probably don't need delete opts.url in parseOptsUrl() at line 103. Should be safe to keep opts.url.

[feross]

For what it's worth, request opted to not fix this issue, i.e. to leave the Host header intact on redirects. But I think it's more sensible to remove it. psf/requests#2366

Fixed in 2.7.1.