refactor/fix: api requests and headers

[michelheusschen]

Centralized login logic in func (*Client) makeLoggedInRequest. Also moved setHeaders to func (*Client) makeRequest and changed some headers that are added:

• Content-Type should only be set when there's a body, so moved to func (c *Client) Login()
• Removed Accept-Encoding, because net/http will handle gzip automatically if the header is unset. The current implementation will fail when the response has a content encoding
• Only set Csrf-Token if there's a token