Return true for Body::is_empty for HEAD responses

[sagebind]

Fixes #341.

[codecov]

Codecov Report

Merging #343 (8a9a5c5) into master (422d4b0) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
- Coverage   75.32%   75.09%   -0.24%     
==========================================
  Files          59       59              
  Lines        3218     3228      +10     
==========================================
  Hits         2424     2424              
- Misses        794      804      +10     

Impacted Files	Coverage Δ
src/client.rs	74.55% <100.00%> (+1.24%)	⬆️
tests/response_body.rs	100.00% <100.00%> (ø)
src/cookies/psl/mod.rs	80.48% <0.00%> (-2.44%)	⬇️
src/agent/mod.rs	70.73% <0.00%> (-2.44%)	⬇️
src/redirect.rs	69.04% <0.00%> (-1.55%)	⬇️
src/config/request.rs	73.97% <0.00%> (-1.37%)	⬇️
testserver/src/mock.rs	79.22% <0.00%> (-1.30%)	⬇️
src/agent/selector.rs	73.25% <0.00%> (-1.17%)	⬇️
src/error.rs	49.71% <0.00%> (-1.15%)	⬇️
src/body/sync.rs	81.18% <0.00%> (-1.00%)	⬇️
... and 6 more

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