URI.unescape handles mixed Unicode/escaped input

Previously, URI.enscape could handle Unicode input (without any actual
escaped characters), or input with escaped characters (but no actual
Unicode characters) - not both.

    URI.unescape("\xe3\x83\x90")  # => "バ"
    URI.unescape("%E3%83%90")  # => "バ"
    URI.unescape("\xe3\x83\x90%E3%83%90")  # =>
                                         # Encoding::CompatibilityError

We need to let `gsub` handle this for us, and then force back to the
original encoding of the input.  The result String will be mangled if
the percent-encoded characters don't conform to the encoding of the
String itself, but that goes without saying.
Signed-off-by: NAshe Connor <ashe@kivikakk.ee>

activesupport/CHANGELOG.md

 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Fix bug where `URI.unscape` would fail with mixed Unicode/escaped character input:
+
+        URI.unescape("\xe3\x83\x90")  # => "バ"
+        URI.unescape("%E3%83%90")  # => "バ"
+        URI.unescape("\xe3\x83\x90%E3%83%90")  # => Encoding::CompatibilityError
+
+    GH#32183
+
+    *Ashe Connor*, *Aaron Patterson*
+
 *   Add `:private` option to ActiveSupport's `Module#delegate`
     in order to delegate methods as private:
 

activesupport/lib/active_support/core_ext/uri.rb

@@ -13,7 +13,7 @@ def unescape(str, escaped = /%[a-fA-F\d]{2}/)
       # YK: My initial experiments say yes, but let's be sure please
       enc = str.encoding
       enc = Encoding::UTF_8 if enc == Encoding::US_ASCII
-      str.gsub(escaped) { |match| [match[1, 2].hex].pack("C") }.force_encoding(enc)
+      str.dup.force_encoding(Encoding::ASCII_8BIT).gsub(escaped) { |match| [match[1, 2].hex].pack("C") }.force_encoding(enc)
     end
   end
 end

activesupport/test/core_ext/uri_ext_test.rb

@@ -9,6 +9,6 @@ def test_uri_decode_handle_multibyte
     str = "\xE6\x97\xA5\xE6\x9C\xAC\xE8\xAA\x9E" # Ni-ho-nn-go in UTF-8, means Japanese.
 
     parser = URI.parser
-    assert_equal str, parser.unescape(parser.escape(str))
+    assert_equal str + str, parser.unescape(str + parser.escape(str).encode(Encoding::UTF_8))
   end
 end