dxpy.DXFile: fix request config logic error in read()
dxpy.DXFile: fix request config logic error in read()

When a read sub-request fails, the request and response iterator get
reset. The logic in read() previously allowed request configuration
from read sub-requests to then leak into the request configuration for
re-initialization of the request and response iterators, which would
then propagate to configuration of the get_download_url
/file-xxxx/download request. This would only manifest when a download
URL needed to be renewed (normally after 24 hours), and result in the
following kind of error traceback in long-running downloads:

  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/scripts/dx.py", line 1757, in cat
    chunk = dxfile.read(1024*1024)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/bindings/dxfile.py", line 653, in read
    content = self._next_response_content()
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/bindings/dxfile.py", line 584, in _next_response_content
    return next(self._response_iterator)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/utils/__init__.py", line 191, in response_iterator
    callable_, args, kwargs = next(request_iterator)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/bindings/dxfile.py", line 562, in _generate_read_requests
    url, headers = self.get_download_url(**kwargs)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/bindings/dxfile.py", line 533, in get_download_url
    resp = dxpy.api.file_download(self._dxid, args, **kwargs)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/api.py", line 909, in file_download
    return DXHTTPRequest('/%s/download' % object_id, input_params, always_retry=always_retry, **kwargs)
  File "/usr/share/dnanexus/lib/python2.7/site-packages/dxpy-0.163.0-py2.7.egg/dxpy/__init__.py", line 373, in DXHTTPRequest
    timeout=timeout, auth=auth, **kwargs)
  File "/usr/bin/../share/dnanexus/lib/python2.7/site-packages/requests/sessions.py", line 451, in request
  ...
MissingSchema: Invalid URL '/file-xxxx/download': No schema supplied. Perhaps you meant http:///file-xxxx/download?

jhuttner/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit