Allow use of Expect header to be configured

[sagebind]

Add a configuration option for changing how POST requests behave under HTTP/1.1 and the Expect: 100-continue request behavior. Allow the user to change the timeout to a different duration or to disable the header entirely.

Replaces #311.

Fixes #303.

[codecov]

Codecov Report

Merging #340 (e0d6b34) into master (1aee4cc) will increase coverage by 0.27%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
+ Coverage   74.80%   75.07%   +0.27%     
==========================================
  Files          58       59       +1     
  Lines        3135     3174      +39     
==========================================
+ Hits         2345     2383      +38     
- Misses        790      791       +1     

Impacted Files	Coverage Δ
src/config/mod.rs	74.12% <95.45%> (+3.87%)	⬆️
src/client.rs	73.30% <100.00%> (+0.38%)	⬆️
src/config/request.rs	75.34% <100.00%> (+0.69%)	⬆️
tests/expect.rs	100.00% <100.00%> (ø)

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