fix: various quirks with how we handle accept headers

[erunion]

🚥 Resolves #697

🧰 Changes

• If we have an accept header in a HAR, and that header is JSON-like, we'll now stop adding it to the code snippet. This'll help us avoid unnecessary bloat, like in the following snippet, because api understands and prioritizes JSON over anything else.

const sdk = require('api')('@basiq/v3.0#edsvz2rlk7p46u2');

sdk.auth('Basic someAuthToken');
sdk.postToken({scope: 'CLIENT_ACCESS'}, {accept: 'application/json', 'basiq-version': '3.0'})
  .then(({ data }) => console.log(data))
  .catch(err => console.error(err));

• If someone does supply an accept header into a code snippet, like the above one, we'll now always send that as a proper header instead of it sometimes getting lumped in with the body payload.
• Fixed a quirk where a header could also be duped and send as a header and in a body payload.

[domharrington]

Lgtm!

[kanadgupta]

few minor questions below but otherwise LGTM