Fix error when adding a time entry and not assigning it to a contract.

[FireMartZ]

Issue #58

[dimvic]

You need to also apply the same change for the create_next_contract private method, I faced the same problem when using this plugin together with Redmine Work Time plugin

  # Create new contract if it is an hourly contract, and the settings configuration
  # is enabled and the hours exceed the current contract.
  private
  def create_next_contract
    return if contract and contract.is_fixed_price
    previous_hours = (hours_was != nil) ? hours_was : 0