Avoid sending superfluous HTTP headers

[blochberger]

See AppPETs/PrivacyService@fdb53ac.

I tried setting:

• request.allHTTPHeaderFields = [:], see URLRequest.allHTTPHeaderFields
• request.setValue(nil, forHTTPHeaderField: "User-Agent"), see URLRequest.setValue(_:forHTTPHeaderField:)
• sessionConfiguration.httpAdditionalHeaders = nil and sessionConfiguration.httpAdditionalHeaders = [:], see URLSessionConfiguration. httpAdditionalHeaders

These methods could be used to override default HTTP headers, but none of them prevents the header from being sent.

The Shalon implementation gives way more control over which HTTP headers are sent.

This issue was migrated from https://gitlab.prae.me/apppets/PrivacyKit/issues/2.