Change the method of detecting if we're creating a new file

Fixes tests on Windows where open('existing-nonempty-file', 'a').tell()
returns 0.  Seriously.

    >>> f = open('newfile.txt', 'a')
    >>> f.tell()
    0
    >>> f.write('hello\n')
    >>> f.tell()
    6
    >>> f.close()
    >>> f = open('newfile.txt', 'a')
    >>> f.tell()
    0                                 # <--- wtf?
    >>> f.write('\n')
    >>> f.tell()
    7