fix: pass empty object to headers initializer to prevent crash on chrome 49

[IlyaSemenov]

ofetch crashes in Chrome 49 (which is the latest available Chrome on Windows XP which is still relevant in certain GovTech) with this message:

Failed to construct 'Headers': No matching constructor signature.

This is because older Chrome didn't treat new Headers(undefined) as new Headers().

This PR fixes this inconvenience with no noticeable burden for modern browsers (or code base readability).

Related: whatwg/fetch#185

[codecov]

Codecov Report

Merging #235 (26a3eda) into main (e27b50d) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
+ Coverage   86.26%   86.29%   +0.03%     
==========================================
  Files           5        5              
  Lines         415      416       +1     
  Branches       68       69       +1     
==========================================
+ Hits          358      359       +1     
  Misses         57       57              

Impacted Files	Coverage Δ
src/fetch.ts	89.27% <100.00%> (+0.04%)	⬆️

[pi0]

Thanks!