Invalid accept header

[aumer-amr]

By default it seems reqwest doesn't specify a correct Accept header, therefore some feeds return the incorrect format.

Example / reproduce:

• Use url = "https://github.com/onedr0p/flux-cluster-template/commits/main/.atom"
• It will fail because it returns a JSON format (blame github).

Possible fix:

• Setting .header(reqwest::header::ACCEPT, "application/atom+xml") on the request will return xml properly, yes this probably won't work with other feed types.

My rust isn't good enough for a full PR, but hopefully this might help or steer in the right direction.

[onedr0p]

I think this might require a new config option to allow people to set request headers.

[morphy2k]

Apparently GitHub has fixed the problem, I can no longer reproduce it. Nevertheless, I have set the Accept header in #178 accordingly to avoid this problem in the future.