Eliminate double download of assets and orders

[sgillies]

#1002 unintentionally introduces a double download. One at https://github.com/planetlabs/planet-client-python/pull/1002/files#diff-eb40861f30b566f7a4a569eb3e052b9edccf30400cbe3a309c34f914aab79cd1R600 and one at https://github.com/planetlabs/planet-client-python/pull/1002/files#diff-eb40861f30b566f7a4a569eb3e052b9edccf30400cbe3a309c34f914aab79cd1R620. The implementation of #1002 only requires the first to be an initial connection to get the Content-length header. We may want to change the implementation, but changing the get to stream is a good enough fix for the moment.