Handle non-ASCII output encoded as ASCII

Some tool involved in `brew install node` is emitting UTF-8 even though
LANG and LC_ALL are unset. Since the external encoding is US-ASCII, we
need to guard against bad data here.

Library/Contributions/cmd/brew-test-bot.rb

@@ -522,7 +522,11 @@ if ARGV.include? "--junit"
       failure = testcase.add_element 'failure' if step.failed?
       if step.has_output?
         # Remove invalid XML CData characters from step output.
-        output = REXML::CData.new step.output.delete("\000\a\b\e\f")
+        output = step.output
+        if output.respond_to?(:force_encoding) && !output.valid_encoding?
+          output.force_encoding(Encoding::UTF_8)
+        end
+        output = REXML::CData.new output.delete("\000\a\b\e\f")
         if step.passed?
           system_out = testcase.add_element 'system-out'
           system_out.text = output