Couple of fixes for various things and making cookie tracking on by default

[quinnj]

These are just a few small fixes. The biggest change is making cookies=true by default, which means we'll track cookies for users automatically. There was at least one case recently where a user was frustrated because it didn't seem like HTTP.jl could do "session management" like they were used to in python requests. We've made a couple of changes recently that make HTTP.jl act more and more like a friendlier client, so I think this makes sense.

cc-ing a couple people in case they have opinions on tracking cookies by default: @staticfloat , @StefanKarpinski , @oxinabox , @mattBrzezinski

[staticfloat]

That's fine with me. Does HTTP also remember HTTP 301 requests?

[codecov-commenter]

Codecov Report

Merging #560 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   77.38%   77.43%   +0.04%     
==========================================
  Files          37       37              
  Lines        2096     2096              
==========================================
+ Hits         1622     1623       +1     
+ Misses        474      473       -1     

Impacted Files	Coverage Δ
src/Handlers.jl	61.42% <100.00%> (ø)
src/cookies.jl	94.33% <100.00%> (ø)
src/StreamRequest.jl	92.45% <0.00%> (-1.89%)	⬇️
src/IOExtras.jl	64.28% <0.00%> (ø)
src/ConnectionPool.jl	78.05% <0.00%> (+0.42%)	⬆️
src/ConnectionRequest.jl	53.48% <0.00%> (+2.32%)	⬆️

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[quinnj]

Ok, I reverted the cookies=true change for now, since I'd like to get more feedback and because I'm trying to get these other bugs fixed quickly.

[oxinabox]

I am a big fan of keeping managing the cookies. Cookie management by hand is just annoying.
You got a cookie, you probably wanted it and needed it for functionality.

[quinnj]

Cool. Just fyi, I took out the cookies=true part of this PR before merging, because there were a bunch of tests that need updating and I needed the other bugfixes for my workshop this morning, but I'll clean it up and do another PR to make cookies auto-handled.