Fix URI#to_s path starting with /

spec/std/uri_spec.cr

@@ -211,6 +211,9 @@ describe "URI" do
     it { URI.new("mailto", path: "[email protected]").to_s.should eq("mailto:[email protected]") }
     it { URI.new("file", path: "/foo.html").to_s.should eq("file:/foo.html") }
     it { URI.new("file", path: "foo.html").to_s.should eq("file:foo.html") }
+    it { URI.new("file", host: "host", path: "foo.html").to_s.should eq("file://host/foo.html") }
+    it { URI.new(path: "//foo").to_s.should eq("/.//foo") }
+    it { URI.new(host: "host", path: "//foo").to_s.should eq("//host//foo") }
 
     it "preserves non-default port" do
       URI.new("http", "www.example.com", 1234).to_s.should eq("http://www.example.com:1234")

src/uri.cr

@@ -147,7 +147,9 @@ class URI
       io << scheme
       io << ':'
     end
-    io << "//" if @user || host || port
+
+    authority = @user || @host || @port
+    io << "//" if authority
     if user = @user
       userinfo(user, io)
       io << '@'
@@ -160,7 +162,16 @@ class URI
     if port = @port
       io << ':' << port
     end
+
+    if authority
+      if !@path.empty? && !@path.starts_with?('/')
+        io << '/'
+      end
+    elsif @path.starts_with?("//")
+      io << "/."
+    end
     io << @path
+
     if query
       io << '?'
       io << query