Allow HTTP.header default to be anything

[omus]

I wanted to be able to set the default to nothing to be able to distinguish from header which is set to the empty string.

[codecov-commenter]

Codecov Report

Merging #820 (134eaee) into master (2cec06e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #820   +/-   ##
=======================================
  Coverage   78.77%   78.77%           
=======================================
  Files          36       36           
  Lines        2502     2502           
=======================================
  Hits         1971     1971           
  Misses        531      531           

Impacted Files	Coverage Δ
src/Messages.jl	86.52% <100.00%> (ø)

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[quinnj]

SGTM; note we currently have a stack of breaking changes on master; so if this is needed in a sooner release, you could propose a branch off the lastest 0.9.X release. I'm hoping to get the breaking release out in the next month or so.

[omus]

SGTM; note we currently have a stack of breaking changes on master; so if this is needed in a sooner release, you could propose a branch off the lastest 0.9.X release. I'm hoping to get the breaking release out in the next month or so.

I'm not in a rush. This has been a minor annoyance and it's good to just know it will be addressed in a future release.