protobuf: Update to 0.6.0-alpha.1

[JohnTitor]

Closes #71

[robjtede]

Any idea what the tarpaulin errors are about?

[JohnTitor]

Any idea what the tarpaulin errors are about?

It's caused by xd009642/tarpaulin#487 and #76 should fix it until the fix is shipped.

[codecov]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #74   +/-   ##
=======================================
  Coverage   78.97%   78.97%           
=======================================
  Files          24       24           
  Lines        2026     2026           
=======================================
  Hits         1600     1600           
  Misses        426      426           

Impacted Files	Coverage Δ
actix-cors/src/lib.rs	83.83% <ø> (ø)

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