fix(ioredis): add default values

Author: dszakallas

lib/instrumentations/ioredis.js

@@ -27,7 +27,7 @@ module.exports = function wrap (redis, agent) {
       return utils.wrapQuery.call(this, original, args, agent, {
         returnsPromise: typeof args[args.length - 1] !== 'function',
         protocol: consts.PROTOCOLS.REDIS,
-        host: host,
+        host: host || 'unknown',
         method: name,
         url: 'unknown'
       })