Use the last redirected URL to create the Gem::Resolver::APISet

In the case that `bundler_api_url` redirects to another URL, this
prevents unnecessary 302s every time the API Resolver requests
dependencies for a gem.

This shaves about 3.5 seconds off the time it takes for me to run
`gem install sinatra`. Even more time is saved when installing gems
with more dependencies.
delegate prefetch to @remote_set in Gem::Resolver::InstallerSet

APISet has a #prefetch method that can request multiple specs at once,
but it was never called, meaning each dependency required a separate
request to the Bundler API.
Merge pull request #773 from charliesome/reduce-api_set-requests

Reduce APISet requests