Append "//" if empty host for file or postgres URI

https://url.spec.whatwg.org/#url-serializing
> Otherwise, if url’s host is null and url’s scheme is "file", append "//" to output.

URL spec doesn't says anything about postgres, but assume the same thing.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60970 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/uri/generic.rb

@@ -1341,7 +1341,7 @@ module URI
       if @opaque
         str << @opaque
       else
-        if @host
+        if @host || %w[file postgres].include?(@scheme)
           str << '//'
         end
         if self.userinfo

test/uri/test_generic.rb

@@ -20,6 +20,10 @@ class URI::TestGeneric < Test::Unit::TestCase
     str = URI(exp).to_s
     assert_equal exp, str
     assert_not_predicate str, :frozen?, '[ruby-core:71785] [Bug #11759]'
+
+    assert_equal "file:///foo", URI("file:///foo").to_s
+    assert_equal "postgres:///foo", URI("postgres:///foo").to_s
+    assert_equal "http:/foo", URI("http:///foo").to_s
   end
 
   def test_parse