Add Accept-Encoding

[lbilli]

What about adding compression by default?

The empty string "" instructs libcurl to add an Accept-Encoding: header containing all built-in supported encodings.

[codecov-io]

Codecov Report

Merging #93 (a1461f9) into master (88a217c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   92.94%   92.96%   +0.01%     
==========================================
  Files           5        5              
  Lines         482      483       +1     
==========================================
+ Hits          448      449       +1     
  Misses         34       34              

Impacted Files	Coverage Δ
src/Curl/Easy.jl	94.11% <100.00%> (+0.02%)	⬆️

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[StefanKarpinski]

It seems that setting this option not only affects what libcurl advertises as accepting but has the side effect of actually doing the decoding before the data is passed to the caller. That might be good but it's certainly worth considering whether we want to do that. It may also be a breaking change since callers that are setting the Accept-Encoding header already and expecting encoded data will no longer get encoded data.