bug fix: #5 fixing writer.py after 1.9.75.123 pull

[vladisld]

There is a regression in test_writer.py test after 1.9.75.123 release/

ERROR: test_writer.test_run

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Traceback (most recent call last):

  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/travis/build/backtrader2/backtrader/tests/test_writer.py", line 48, in test_run
    writer=(bt.WriterStringIO, dict(csv=True)))
  File "/home/travis/build/backtrader2/backtrader/tests/testcommon.py", line 110, in runtest
    cerebro.run()
  File "/home/travis/build/backtrader2/backtrader/backtrader/cerebro.py", line 1085, in run
    wr = wrcls(*wrargs, **wrkwargs)
  File "/home/travis/build/backtrader2/backtrader/backtrader/metabase.py", line 88, in __call__
    _obj, args, kwargs = cls.doinit(_obj, *args, **kwargs)
  File "/home/travis/build/backtrader2/backtrader/backtrader/metabase.py", line 78, in doinit
    _obj.__init__(*args, **kwargs)
  File "/home/travis/build/backtrader2/backtrader/backtrader/writer.py", line 218, in __init__
    self.out = self.out()
AttributeError: 'WriterStringIO' object has no attribute 'out'

The problem is with removed initialization of out member in init method of WriterFile. The member is a actually expected to be initialized in init method - see the implementation of the WriterStringIO.

The possible simple fix, is to move the self.out = ... initialization in WriterStringIO.init to WriterStringIO.start method