use Object.assign instead of spread operators

Author: F4bio16

lib/log.js

@@ -47,9 +47,9 @@ const _loggersByName = {}
  * parent's options.
  */
 class MonkLogger extends logLevel.constructor {
-  constructor (name, level, options) {
+  constructor (name, level, options = {}) {
     super(name, level)
-    this.options = {...defaultLogLevelOptions}
+    this.options = Object.assign({}, options, defaultLogLevelOptions)
     this.configure(options)
   }
 
@@ -66,10 +66,7 @@ class MonkLogger extends logLevel.constructor {
    *   options to defaults.
    */
   configure (options, reset = false) {
-    this.options = {
-      ...(reset ? defaultLogLevelOptions : this.options),
-      ...options,
-    }
+    this.options = Object.assign({}, (reset ? defaultLogLevelOptions : this.options), options)
     prefix.apply(this, this.options)
   }
 
@@ -88,7 +85,7 @@ class MonkLogger extends logLevel.constructor {
     let logger = _loggersByName[name]
     if (!logger) {
       logger = _loggersByName[name] = new MonkLogger(
-        name, level || this.getLevel(), {...this.options, ...options})
+        name, level || this.getLevel(), Object.assign({}, this.options, options))
     }
     return logger
   }