Fix HTTP.download stack overflow error on "/" paths

[quinnj]

No description provided.

[quinnj]

cc: @oxinabox. I'm not super familiar with the download code, but this seems to be in the same spirit as what file_from_target is aiming to do. I'll try to figure out if I can see how we usually right tests for download stuff as well.

[codecov-commenter]

Codecov Report

Merging #897 (d2f58be) into master (cf706bc) will increase coverage by 0.10%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #897      +/-   ##
==========================================
+ Coverage   80.12%   80.22%   +0.10%     
==========================================
  Files          36       36              
  Lines        2878     2903      +25     
==========================================
+ Hits         2306     2329      +23     
- Misses        572      574       +2     

Impacted Files	Coverage Δ
src/ConnectionPool.jl	89.21% <ø> (+0.27%)	⬆️
src/Messages.jl	85.54% <0.00%> (-2.12%)	⬇️
src/download.jl	0.00% <0.00%> (ø)
src/clientlayers/ConnectionRequest.jl	78.94% <90.00%> (+2.32%)	⬆️
src/HTTP.jl	85.33% <100.00%> (+2.28%)	⬆️
src/Handlers.jl	84.14% <100.00%> (+0.29%)	⬆️
src/clientlayers/RetryRequest.jl	75.60% <0.00%> (-1.14%)	⬇️
... and 1 more

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[oxinabox]

I'll try to figure out if I can see how we usually right tests for download stuff as well.

We usually write tests using http://test.greenbytes.de/tech/tc2231/ which is like reference API for this particular part of the HTTP spec.

I think this one actually corresponds to the absolute path part of spec which is tested at:
http://test.greenbytes.de/tech/tc2231/#attabspath

For which the correct behavour is:

Either ignore the filename altogether, or discard the path information.

Though all web browsers:

Replaces the path separator with "_".

so we might want to match that.