Handle ETag and If-None-Match

[leo]

This adds native support for 304 responses for any path in the case that:

• A ETag header was set for that path
• A request comes in to that path with If-None-Match matching the previously sent ETag

Only if both are true, the 304 response will occur.

[codecov]

Codecov Report

Merging #39 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #39   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         285    289    +4     
=====================================
+ Hits          285    289    +4

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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