fix: check for existence of fetch before using it

[JustinBeckwith]

This may fix googleapis/nodejs-storage#730

[codecov]

Codecov Report

Merging #138 into master will decrease coverage by 0.98%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
- Coverage   92.24%   91.25%   -0.99%     
==========================================
  Files           5        4       -1     
  Lines         580      572       -8     
  Branches      119      120       +1     
==========================================
- Hits          535      522      -13     
- Misses         43       48       +5     
  Partials        2        2

Impacted Files	Coverage Δ
src/gaxios.ts	85.82% <42.85%> (-1.89%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2db0f66...25654d8. Read the comment docs.