fix: remove headers with undefined value

[aarongranick-okta]

In 4.0 we switched from util.extend to Object.assign. The side-effect is that "undefined" headers which were being removed are now being passed with the value "undefined".

• Adds unit tests for HTTP requestor

[codecov-commenter]

Codecov Report

Merging #468 into master will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #468      +/-   ##
==========================================
+ Coverage   90.13%   90.47%   +0.33%     
==========================================
  Files          31       31              
  Lines        1561     1574      +13     
  Branches      342      344       +2     
==========================================
+ Hits         1407     1424      +17     
+ Misses        154      150       -4     

Impacted Files	Coverage Δ
lib/OktaAuthBase.ts	82.35% <ø> (ø)
lib/browser/browser.ts	93.06% <ø> (ø)
lib/TokenManager.ts	97.02% <100.00%> (+0.22%)	⬆️
lib/http.ts	100.00% <100.00%> (+3.38%)	⬆️
lib/server/serverStorage.ts	100.00% <0.00%> (+10.52%)	⬆️

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[aarongranick-okta]

@shuowu @swiftone This should have gone in with 4.0.1 but it was forgotten. Maybe we can get this in for 4.0.2. The regression was observed in the SIW, which has not switched to auth-js 4.0 yet.