Update client.go - allow to skip Authorization header

[7flash]

Currently I am hosting my LiteLLM proxy with no Authorization and trying to connect with go-openai like this:

	config := openai.DefaultConfig("")
	config.BaseURL = "https://my-service/v1"
	aiClient := openai.NewClientWithConfig(config)

It shows an error because I am passing empty token but it still appends Bearer header

2024/02/12 12:51:54 Failed to create chat stream: error, status code: 401, message: Authentication Error,
^Csignal: interrupt

While not passing any authorization header works for my proxy server

[sashabaranov]

@7flash please rebase your PR, I've just fixed all the linter warnings

[sashabaranov]

@7flash okay, there's still some linter issue to be fixed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (11ad4b6) 98.44% compared to head (48c4fd1) 98.44%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #658   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files          24       24           
  Lines        1347     1347           
=======================================
  Hits         1326     1326           
  Misses         15       15           
  Partials        6        6           

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