fix(extension/fetch): Fetching with method HEAD should not have body

[getspooky]

HEAD and OPTIONS requests should not have bodies https://fetch.spec.whatwg.org/#concept-main-fetch

[getspooky]

Current behaviour

const response = await fetch("https://api.github.com/users/denoland", {
  method: "HEAD",
});
// Result 
/*
Response {
  body: null,
  bodyUsed: false,
  headers: Headers {
  "accept-ranges": "bytes",
  "access-control-allow-origin": "*",
  "access-control-expose-headers": "ETag, Link, Location, Retry-After, X-GitHub-OTP, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLim...",
  "cache-control": "public, max-age=60, s-maxage=60",
  "content-security-policy": "default-src 'none'",
  "content-type": "application/json; charset=utf-8",
  date: "Wed, 16 Jun 2021 11:35:08 GMT",
  etag: 'W/"456078ee283e268960ebf9ee2fbdd5f8f40cd215f8811a5b0917b05b49fefcd1"',
  "last-modified": "Wed, 31 Mar 2021 14:25:32 GMT",
  "referrer-policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
  server: "GitHub.com",
  "strict-transport-security": "max-age=31536000; includeSubdomains; preload",
  vary: "Accept, Accept-Encoding, Accept, X-Requested-With",
  "x-content-type-options": "nosniff",
  "x-frame-options": "deny",
  "x-github-media-type": "github.v3; format=json",
  "x-github-request-id": "8924:60BE:202550:24EC32:60C9E1EC",
  "x-ratelimit-limit": "60",
  "x-ratelimit-remaining": "57",
  "x-ratelimit-reset": "1623846694",
  "x-ratelimit-resource": "core",
  "x-ratelimit-used": "3",
  "x-xss-protection": "0"
},
  ok: true,
  redirected: false,
  status: 200,
  statusText: "OK",
  url: "https://api.github.com/users/denoland"
}
*/

[lucacasonato]

LGTM, thanks