class ActiveSupport::Cache::MemCacheStore

def escape_key(key)

characters properly.
before applying the regular expression to ensure we are escaping all
Memcache keys are binaries. So we need to force their encoding to binary 
def escape_key(key)
  key = key.to_s.dup
  key = key.force_encoding(Encoding::ASCII_8BIT)
  key = key.gsub(ESCAPE_KEY_CHARS){ |match| "%#{match.getbyte(0).to_s(16).upcase}" }
  key = "#{key[0, 213]}:md5:#{Digest::MD5.hexdigest(key)}" if key.size > 250
  key
end