Avoid logging mulitple times for performance gains.

Before: 
![screenshot from 2014-07-07 09 29 06](https://cloud.githubusercontent.com/assets/4335742/3490656/21041f8c-0576-11e4-8c75-83929fb09767.png)

After: 
![screenshot from 2014-07-07 09 29 12](https://cloud.githubusercontent.com/assets/4335742/3490657/2148180e-0576-11e4-9847-19daaa1b5850.png)

@senny Ah ok I see what you mean. Since Rails by default defines a [custom formatter](https://github.com/rails/rails/blob/b40faf423cb032cf326e07e7c7bd578c9052b282/activesupport/lib/active_support/logger.rb#L53) which strips away all the extra information, perhaps we might not have to concern ourselves with the above? (Not big on this just wanted to get your thoughts) :smile: Either way I think we should clear up the discrepancy [here](https://github.com/rails/rails/blob/21a49ffe9889a351882e8e97fc45790717b5a156/actionmailer/lib/action_mailer/log_subscriber.rb#L11) 

```ruby
2.1.1 :001 > require 'logger'
 => true 
2.1.1 :002 > l = Logger.new(STDOUT)
 => #<Logger:0x000000014682e0 @progname=nil, @level=0, @default_formatter=#<Logger::Formatter:0x00000001468290 @datetime_format=nil>, @formatter=nil, @logdev=#<Logger::LogDevice:0x00000001468240 @shift_size=nil, @shift_age=nil, @filename=nil, @dev=#<IO:<STDOUT>>, @mutex=#<Logger::LogDevice::LogDeviceMutex:0x000000014681f0 @mon_owner=nil, @mon_count=0, @mon_mutex=#<Mutex:0x000000014681a0>>>> 
2.1.1 :003 > l.info "\n1"
I, [2014-07-17T00:35:54.803536 #5431]  INFO -- : 
1
 => true 
```