Fix to_s in AdApiFaultDetail

Author: jplopez

lib/bing-ads-api/fault/ad_api_fault_detail.rb

@@ -31,7 +31,7 @@ def initialize(attributes={})
 		# Author:: jlopezn@neonline.cl 
 		def to_s
 			str = super.to_s + ":\n" 
-			if batch_errors
+			if errors
 				str += "\tErrors:\n" + errors.map{ |e| "\t" + e.to_s }.join("\n")
 			end