Fix where redirecting URLs may not include schemes or hosts

[belphegor-belbel]

Section 7.1.2 in RFC-7231 defines a "Location:" header for specifying the destination URL,
it may not contain schemes or hosts as shown in the example ("Location: /People.html#tim").

https://www.rfc-editor.org/rfc/rfc7231

However in NSISdl current implementation, the Location header's content is passed to the
connect() method as-is, so the first part up to "/" is interpreted as the host name,
so it fails.

This patch makes it possible to regenerate the destination URL from the original one.

[sredna]

• Hardcodes buffer size of 1024
• Does not support //host/path relative URLs?