Get tests passing with frozen-string-literals enabled.

Though this does require patches for test-unit, as covered in test-unit/test-unit#149.

Author: pat

lib/redis/connection/ruby.rb

@@ -39,7 +39,7 @@ def initialize(*args)
         super(*args)
 
         @timeout = @write_timeout = nil
-        @buffer = ""
+        @buffer = "".dup
       end
 
       def timeout=(timeout)