Inline write_rfc2822_inner, don't localize

[pitdicker]

As discussed in #1144 (comment) we should not localize the day and month names in write_rfc2822 with the unstable-locales feature, because the RFC requires English names.

[codecov]

Codecov Report

Merging #1322 (096eb5f) into 0.4.x (be8f2fd) will decrease coverage by 0.01%.
Report is 9 commits behind head on 0.4.x.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##            0.4.x    #1322      +/-   ##
==========================================
- Coverage   91.25%   91.24%   -0.01%     
==========================================
  Files          38       38              
  Lines       17114    17107       -7     
==========================================
- Hits        15617    15610       -7     
  Misses       1497     1497              

Files	Coverage Δ
src/format/formatting.rs	92.46% <88.88%> (-0.08%)	⬇️

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