Hit false precondition when calling `HTTPRequest(url:)` with an URL which has base URL

[li-bei]

let baseURL = URL(string: "https://foo.com")!
let url = URL(string: "/bar", relativeTo: baseURL)!
let httpRequest = HTTPRequest(url: url)

will hit precondition in HTTPRequest+URL.swift:103:

precondition(schemeRange.location != kCFNotFound, "Schemeless URL is not supported")

[guoye-zhang]

Should be addressed in 0.2.1