Inconsistent Addressable::URI#heuristic_parse behavior with schemaless URLs

[dwbutler]

I would expect Addressable::URI#heuristic_parse to give the same results for the completely schemaless test.com and the protocol-relative schemaless url //test.com

Expected:

uri = Addressable::URI.heuristic_parse("//test.com")
uri.scheme
=> "http"
uri.relative?
=> false

Actual:

uri = Addressable::URI.heuristic_parse("//test.com")
uri.scheme
=> nil
uri.relative?
=> true

[sporkmonger]

Good point. I agree.

[dentarg]

From #401 (comment)

I think it's worth calling out that Addressable's heuristic_parse does aim to hew quite closely to whatever browsers are doing in the address bar.

If I enter //test.com in Google Chrome it tries to visit file:///test.com. Firefox tries to visit file:////test.com. Safari does a Google search for //test.com 😅

[dentarg]

Related to #356