prepare release beta 4

[robjtede]

i think this will be the last beta 🚀

[codecov-commenter]

Codecov Report

Merging #1659 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1659      +/-   ##
==========================================
- Coverage   53.35%   53.28%   -0.07%     
==========================================
  Files         117      121       +4     
  Lines       11813    11828      +15     
==========================================
  Hits         6303     6303              
- Misses       5510     5525      +15     

Impacted Files	Coverage Δ
actix-http/src/header/common/mod.rs	10.00% <0.00%> (-6.67%)	⬇️
actix-http/src/header/common/accept.rs	0.00% <0.00%> (ø)
...ctix-http/src/header/common/if_unmodified_since.rs	0.00% <0.00%> (ø)
actix-http/src/header/common/etag.rs	0.00% <0.00%> (ø)
actix-http/src/header/common/if_match.rs	0.00% <0.00%> (ø)
actix-http/src/header/common/accept_charset.rs	0.00% <0.00%> (ø)

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[Neopallium]

looks good.

[popzxc]

Can it wait for #1656 to get merged, so that new version will have better docs already?