Add test for HTTP::Request with resource string //

[miry]

While testing the awscr-signer library job, I noticed a change in the behavior of HTTP::Request#path when handling the case of //:

Old behavior:

request = HTTP::Request.new("GET", "//", HTTP::Headers.new)
request.path.should eq("/")

New behavior:

request = HTTP::Request.new("GET", "//", HTTP::Headers.new)
request.path.should eq("//")

I am trying to add more tests to cover changes from #15499

[straight-shoota]

I think going from / to // in #15499 is actually a welcome change. Request#path shouldn't worry about URI semantics. It just needs to represent the request's original path as accurately as possible.

I might even question whether it makes a lot of sense to parse this through URI. Instead, it could just return the raw request target, capped at the first ? character. But that's a separate discussion.

So anyway for this PR I think we should just change the expected value to match the actual one and then we're good.