complete uri scheme from referer_uri

spec/http_spec.cr

@@ -4,6 +4,8 @@ WebMock.stub(:post, "http://example.com/post")
   .with(body: "email=foobar", headers: {"Content-Type" => "application/x-www-form-urlencoded"})
   .to_return(body: "success")
 WebMock.stub(:get, "example.com/%E3%81%82%E3%81%82%E3%81%82")
+WebMock.stub(:get, "https://example.com/")
+WebMock.stub(:get, "https://example.com/post")
 
 describe "Mechanize HTTP test" do
   it "simple GET" do
@@ -62,4 +64,11 @@ describe "Mechanize HTTP test" do
     page = agent.get("http://example.com/")
     agent.request_headers["User-Agent"].should eq mac_chrome_agent
   end
+
+  it "can complete uri when uri is relative" do
+    agent = Mechanize.new
+    agent.get("https://example.com/")
+    page = agent.get("/post")
+    page.uri.to_s.should eq "https://example.com/post"
+  end
 end

src/mechanize/http/agent.cr

@@ -151,10 +151,16 @@ module MechanizeCr
         if target_url.host.nil? && referer && referer_uri.try &.host
           target_url.host = referer_uri.not_nil!.host
         end
+
         # fill scheme if scheme isn't set
         if target_url.relative?
-          target_url.scheme = "http"
+          if referer && referer_uri.try &.scheme
+            target_url.scheme = referer_uri.not_nil!.scheme
+          else
+            target_url.scheme = "http"
+          end
         end
+
         # fill path's slash if there's no slash.
         if target_url.path && (target_url.path.empty? || target_url.path[0] != '/')
           target_url.path = "/#{target_url.path}"