assertions.rb: fallback encoding error

* test/lib/test/unit/assertions.rb (message): fallback when
  outputs from different encoding commands mixed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52897 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -490,7 +490,11 @@ EOT
             if 1 < ary.length
               ary[0...-1] = ary[0...-1].map {|str| str.sub(/(?<!\.)\z/, '.') }
             end
+            begin
               ary.join("\n")
+            rescue Encoding::CompatibilityError
+              ary.map(&:b).join("\n")
+            end
           end
         else
           super