set UTF-8 if given URI string is ASCII

Now URI is normally UTF-8, and US-ASCII URI string is considered as
escaped a UTF-8 string.
https://github.com/rails/rails/issues/32294

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62897 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/uri/rfc2396_parser.rb

@@ -329,7 +329,9 @@ module URI
     # Removes escapes from +str+
     #
     def unescape(str, escaped = @regexp[:ESCAPED])
-      str.gsub(escaped) { [$&[1, 2]].pack('H2').force_encoding(str.encoding) }
+      enc = str.encoding
+      enc = Encoding::UTF_8 if enc == Encoding::US_ASCII
+      str.gsub(escaped) { [$&[1, 2]].pack('H2').force_encoding(enc) }
     end
 
     @@to_s = Kernel.instance_method(:to_s)

test/uri/test_parser.rb

@@ -50,6 +50,7 @@ class URI::TestParser < Test::Unit::TestCase
     p1 = URI::Parser.new
     assert_equal("\xe3\x83\x90", p1.unescape("\xe3\x83\x90"))
     assert_equal("\xe3\x83\x90", p1.unescape('%e3%83%90'))
+    assert_equal("\u3042", p1.unescape('%e3%81%82'.force_encoding(Encoding::US_ASCII)))
     assert_equal("\xe3\x83\x90\xe3\x83\x90", p1.unescape("\xe3\x83\x90%e3%83%90"))
   end
 end