Allow to remove headers

[wellingguzman]

This creates the opportunity to delete any header when the value is null.

Example below will remove the default Content-Disposition:

{
  "headers": [
    {
      "source" : "**/*.*",
      "headers" : [{
        "key" : "Content-Disposition",
        "value" : null
      }]
    }]
}

Ref: vercel/serve#438

[codecov]

Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #45   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         305    309    +4     
=====================================
+ Hits          305    309    +4

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

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[leo]

Thanks!