Skip empty chucks for BodyStream and SizedStream

[tyranron]

Fixes #1267

This PR makes BodyStream and SizedStream in actix-http crate to skip empty chunks whicl streaming HTTP response, so omit unexpected termination.

[tyranron]

@JohnTitor I've implemented option 2 described here as the most preferred among commenters. But I'll change it if we should go with another one.

[codecov-io]

Codecov Report

Merging #1308 into master will decrease coverage by <.01%.
The diff coverage is 71.87%.

@@            Coverage Diff             @@
##           master    #1308      +/-   ##
==========================================
- Coverage   79.43%   79.43%   -0.01%     
==========================================
  Files         146      146              
  Lines       17004    17006       +2     
==========================================
+ Hits        13507    13508       +1     
- Misses       3497     3498       +1

Impacted Files	Coverage Δ
src/test.rs	83.96% <ø> (ø)	⬆️
actix-http/src/client/pool.rs	42.85% <0%> (-0.19%)	⬇️
src/extract.rs	86.2% <100%> (+0.24%)	⬆️
actix-http/src/h2/dispatcher.rs	71.25% <83.33%> (ø)	⬆️

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[JohnTitor]

Could you also update the changelog with the PR number?

[JohnTitor]

Thanks!