fix: allow optional body for mock reply

[JustinBeckwith]

Quick quality of life fix. The reply method today on the mock interceptor requires both an HTTP status code and a body to send. In a lot of cases, it's common to mock out an empty body. Trying to make this as smooth as nock by defaulting the body to '' :)

[codecov-commenter]

Codecov Report

Merging #1536 (1cb06f0) into main (c1a0490) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1536      +/-   ##
==========================================
- Coverage   94.90%   94.87%   -0.03%     
==========================================
  Files          50       50              
  Lines        4608     4609       +1     
==========================================
  Hits         4373     4373              
- Misses        235      236       +1     

Impacted Files	Coverage Δ
lib/mock/mock-interceptor.js	98.66% <100.00%> (-1.34%)	⬇️
lib/fetch/body.js	98.51% <0.00%> (-0.02%)	⬇️
lib/fetch/webidl.js	98.29% <0.00%> (+<0.01%)	⬆️
lib/fetch/util.js	82.35% <0.00%> (+0.17%)	⬆️

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[mcollina]

lgtm