Handle different VM encoding cases.

Unconvertible characters are replaced with literal '?' characters.
Note that in unicode and other character encodings there are multiple
'?' characters, but the one I'm talking about is the one that you'd get
if you took ascii and transcoded it to whichever character encoding
you're working in.

lib/rspec/expectations/encoded_string.rb

@@ -25,7 +25,17 @@ def split(regex_or_string)
         def matching_encoding(string)
           string.encode(encoding)
         rescue Encoding::UndefinedConversionError
-          string.encode(encoding, :undef => :replace)
+          normalize_missing(string.encode(encoding, :invalid => :replace, :undef => :replace))
+        rescue Encoding::ConverterNotFoundError
+          normalize_missing(string.force_encoding(encoding).encode(:invalid => :replace))
+        end
+
+        def normalize_missing(string)
+          if encoding.to_s == "UTF-8"
+            string.gsub("\xEF\xBF\xBD".force_encoding(encoding), "?")
+          else
+            string
+          end
         end
       else
         def matching_encoding(string)

spec/rspec/expectations/encoded_string_spec.rb

@@ -17,13 +17,15 @@ module RSpec::Expectations
         end
 
         context 'with a string that cannot be converted to the target encoding' do
-          it 'replaces undefined characters' do
+          it 'replaces undefined characters with either a ? or a unicode ?' do
             ascii_string = "\xAE".force_encoding("ASCII-8BIT")
             valid_unicode_string = "\xE2\x82\xAC".force_encoding('UTF-8')
 
             resulting_string = build_encoded_string(valid_unicode_string, target_encoding) << ascii_string
-            expected_bytes = [226, 130, 172, 239, 191, 189]
-            expect(resulting_string.each_byte.to_a).to eq expected_bytes
+            expected_bytes = [226, 130, 172, "?".unpack("c").first]
+            actual_bytes = resulting_string.each_byte.to_a
+
+            expect(actual_bytes).to eq(expected_bytes)
           end
         end