feat(restLink): add Headers polyfill when Headers is undefined

[leoswing]

Add Headers polyfill when Headers is undefined in some special browser environment like weixin.
It does not has any related to window or Headers.

• feature

[apollo-cla]

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[codecov-io]

Codecov Report

Merging #225 into master will decrease coverage by 0.2%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   92.71%   92.51%   -0.21%     
==========================================
  Files           3        3              
  Lines         412      414       +2     
  Branches      123      124       +1     
==========================================
+ Hits          382      383       +1     
- Misses         28       29       +1     
  Partials        2        2

Impacted Files	Coverage Δ
src/restLink.ts	92.26% <50%> (-0.22%)	⬇️

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