Fixed NaN `:rate` and 0.0 `:eta` when `curr` is set.

[francisokoyo]

• Setting the current progress index to anything other than 0 will cause the value of this.start to be undefined. This causes the calculation for :eta and :rate to produce useless information.
  • Replaced conditional to set start only when the progress is ticked when current == 0.
  • We will always set the start time on first tick anyway.

This PR should resolve and close #159