Do not fail in Loga::Event#to_s when there is no timestamp

[bliof]

No description provided.

[bliof]

There is also some problem with timecop config when I run the specs locally:

  2) Loga::Event#to_s when no exception outputs the message
     Failure/Error: expect(subject.to_s).to eql("#{time_anchor.iso8601(3)} Hello World")

       expected: "2015-12-15T09:30:05.123+06:00 Hello World"
            got: "2015-12-15T05:30:05.123+02:00 Hello World"

[timstott]

@bliof in what situation is a Loga event instantiated without a timestamp?

[bliof]

I had problems with it in #79

but here is an example

require 'logger'
require 'loga'

logger = Logger.new($stdout)
event = Loga::Event.new

logger.info(event)

Should the timestamp of the event default to current time?

[timstott]

@bliof Good example, I understand now. Overriding #to_s was a hack to output the timestamp in :simple formatter. Ultimately I want to remove the override and have the a simple formatter in Loga output the timestamp.

[timstott]

Fixed in c794943 in #92