Default value for headers

[sm2017]

As headers 's default value is undefined , We cannot do the following

const error = new TooManyRequestsError();
error.headers['Retry-After'] = 120;

We have Cannot set property 'Retry-After' of undefined error

[sm2017]

Please release a version after accepting this PR

[codecov]

Codecov Report

Merging #4 into master will decrease coverage by 54.96%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master       #4       +/-   ##
===========================================
- Coverage    99.7%   44.73%   -54.97%     
===========================================
  Files          50       50               
  Lines         341      342        +1     
  Branches       47       47               
===========================================
- Hits          340      153      -187     
- Misses          0      189      +189     
+ Partials        1        0        -1

Impacted Files	Coverage Δ
lib/http/http_error.ts	28.57% <0%> (-54.77%)	⬇️
lib/base.ts	20% <0%> (-80%)	⬇️
lib/http/413.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/503.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/418.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/422.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/423.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/428.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/502.ts	33.33% <0%> (-66.67%)	⬇️
lib/http/504.ts	33.33% <0%> (-66.67%)	⬇️
... and 38 more

---

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[popomore]

Can you add a testcase?

[sm2017]

Added

[popomore]

I don't see the testcase in your commit.

[sm2017]

@popomore Added , Check now

[popomore]

CI is failed, so I commited by myself, Thanks for you PR.

REF: #6