Fixed file upload error in python3

[mmasashi]

I got the following error on file upload when running the server process in python3.

  File "....../rest_framework_proxy/utils.py", line 61, in build_multipart_header
    return b'\r\n'.join(output)
TypeError: sequence item 0: expected a bytes-like object, str found

This happens because output is not binary string. I made changes to encode string into binary string in the utils module.

[codecov-io]

Codecov Report

Merging #26 into master will increase coverage by 17.33%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #26       +/-   ##
===========================================
+ Coverage   50.22%   67.55%   +17.33%     
===========================================
  Files           5        5               
  Lines         225      225               
===========================================
+ Hits          113      152       +39     
+ Misses        112       73       -39

Impacted Files	Coverage Δ
rest_framework_proxy/utils.py	88.88% <100%> (+66.66%)	⬆️
rest_framework_proxy/views.py	71.11% <0%> (+6.66%)	⬆️

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[eofs]

Thanks, merged! 👍