added support for uppercase version of http proxy environment variables

[racinmat]

Fixes #646

[codecov-io]

Codecov Report

Merging #648 (b9e98d0) into master (edc5e28) will decrease coverage by 0.17%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #648      +/-   ##
==========================================
- Coverage   77.63%   77.45%   -0.18%     
==========================================
  Files          36       36              
  Lines        2325     2329       +4     
==========================================
- Hits         1805     1804       -1     
- Misses        520      525       +5     

Impacted Files	Coverage Δ
src/HTTP.jl	100.00% <ø> (ø)
src/ConnectionRequest.jl	46.87% <50.00%> (-1.46%)	⬇️
src/RetryRequest.jl	55.00% <0.00%> (-5.00%)	⬇️
src/Messages.jl	92.48% <0.00%> (-0.76%)	⬇️

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