fix: use http.Request's GetBody

[mstoykov]

This is needed for correct handling of redirects of requests with bodies
and in http2 when the server sends GoAway without an error.

Unfortunately testing the GoAway case turned out to be problematic so it
is left for the future.

Fixes #1091

[na--]

LGTM, though don't merge it for now 😄

[codecov-io]

Codecov Report

Merging #1093 into master will increase coverage by 0.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #1093      +/-   ##
==========================================
+ Coverage    73.1%   73.12%   +0.01%     
==========================================
  Files         139      139              
  Lines       10223    10228       +5     
==========================================
+ Hits         7474     7479       +5     
  Misses       2308     2308              
  Partials      441      441

Impacted Files	Coverage Δ
lib/netext/httpext/request.go	89.19% <92.85%> (+0.19%)	⬆️

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[na--]

Can you rebase on the new master, and also add a line for this in the upcoming release notes?