Do not read content type from redirect responses

[tlaundal]

The content type header is irrelevant for redirect (status 300 to 400) responses, as they do not say anything about the content type of the actual resource they redirect to.

This commit makes the extension silently ignore redirect responses. The resource at the end of the redirect is still handled properly.

No special consideration is taken for status code 300 which will not make the browser create a new request, as the response body for these responses should already be made for display to the user.

Fixes #172

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The following example server reproduces the problem described in #172, and also reveals a problem for the /redirect-json/json route. The PR fixes both the issues. I have only tested the issues and solution in Firefox 79.

const app = require('express')();

const redirect_json = Location => (_, res) =>
  res.status(303).set({'Content-Type': 'application/json', Location}).send();

app.get('/text', (_, res) => res.send('Text')); 
app.get('/json', (_, res) => res.send({'json': 'object'}));
app.get('/redirect/text', (_, res) => res.redirect('/text'));
app.get('/redirect/json', (_, res) => res.redirect('/json'));
app.get('/redirect-json/text', redirect_json('/text'));
app.get('/redirect-json/json', redirect_json('/json'));

app.listen(3000);

[bhollis]

Awesome, thank you for doing this!