Fix custom logger assignment (#9)

* In `consumer.go` the `NewConsumer` function was testing if the `Logger` attribute
  on the Config struct is `nil` and if it was it assigned the consumer
  logger to `&defaultLogger{}` but if the Config.Logger is not nil its value
  was never getting set on the consumer leaving consumer.logger == nil

* In the `Logger` method on 109 there is a check for whether
  `consumer.logger == nil` and if it is we return `&defaultLogger{}`

* Combined these to pieces of logic result in always ignoring a custom
  Logger attribute when it is set on Config.

* This switches the logic in `NewConsumer` to match the pattern for
  other optional attributes by checking if the `Logger` attribute on
  the provided config is `!= nil` and setting consumer's logger to
  the provided logger

* If there is no logger assigned on Config `consumer.logger` will
  remain `nil` and the `Logger` function will dutifly return a
  `&defaultLogger{}`

Author: joshcass

consumer.go

@@ -75,8 +75,8 @@ func NewConsumer(c Config, queueName string) (Consumer, error) {
 		extensionLimit:    2,
 	}
 
-	if c.Logger == nil {
-		cons.logger = &defaultLogger{}
+	if c.Logger != nil {
+		cons.logger = c.Logger
 	}
 
 	if c.VisibilityTimeout != 0 {