Fix issue where attempting to download a binary file fails

[penn5]

It occured because the file is opened in 'r' mode and .encode("utf-8")'d rather than just opening in 'rb' mode to start with. Sometimes a 'r' file may be provided, so we have to maintain compatibility with this case.

[coveralls]

Coverage remained the same at 43.623% when pulling 6a627fc on penn5:patch-2 into 0e50795 on google:master.

[penn5]

Any updates? I just had to guide a user through installing this branch because they encountered this issue.

[fvguilherme]

Any updates? I just had to guide a user through installing this branch because they encountered this issue.

I can confirm this. I got this:

File "/usr/lib/python3.7/site-packages/adb/fastboot.py", line 291, in FlashFromFile
  progress_callback=progress_callback)
File "/usr/lib/python3.7/site-packages/adb/fastboot.py", line 326, in Download
  source_file, source_len, info_cb, progress_callback=progress_callback)
File "/usr/lib/python3.7/site-packages/adb/fastboot.py", line 137, in HandleDataSending
  self._Write(source_file, accepted_size, progress_callback)
File "/usr/lib/python3.7/site-packages/adb/fastboot.py", line 195, in _Write
  tmp = data.read(self.chunk_kb * 1024)
File "/usr/lib/python3.7/codecs.py", line 322, in decode
  (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xbd in position 2: invalid start byte