Fixed content error when response is binary file

[vincentwan]

With return_raw set to true, I got wrong content in the response data when trying to proxy an API that returns an mp3 file.

Seems that we are using the response.text field even for binary content.
And what happens inside response.text is:

content = str(self.content, errors='replace')

And then we encode the result string with utf-8.

Fixed that by setting the content field instead in the HttpResponse.

A test case for the return raw case is provided.

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 0.88%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   67.55%   68.44%   +0.88%     
==========================================
  Files           5        5              
  Lines         225      225              
==========================================
+ Hits          152      154       +2     
+ Misses         73       71       -2

Impacted Files	Coverage Δ
rest_framework_proxy/views.py	72.59% <100%> (+1.48%)	⬆️

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[vincentwan]

Hi @eofs could you please see if this is good to merge? Thanks!