Authorization header added multiple times

**Describe the bug**
If the incoming request to the content scanner includes `Authorization` header but the config file also has a hardcoded one (using `additional_headers` option) or it is being asked to forward (using `headers_to_forward`) then the header gets added to the upstream request multiple times making it fail, without any clear trace

**To Reproduce**
Steps to reproduce the behavior:
1. Configure a hardcoded `Authorization` header in config with `additional_headers`.
2. Send a request to content scanner with `Authorization` header.

or

1. Configure the `Authorization` header in config to be forwarded with `headers_to_forward`.
2. Send a request to content scanner with `Authorization` header.

**Expected behavior**
At all times, use at most one copy of the `Authorization` header in the upstream request. If there is a hardcoded one in the config (`additional_headers`), use this one instead of one in the incoming request - and indicate that in the logs.

**Actual behavior**
The request fails, there is nothing in the logs which could help you to debug the issue.

**Screenshots**
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**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

**Smartphone (please complete the following information):**
- Device: [e.g. iPhone6]
- OS: [e.g. iOS8.1]
- Browser [e.g. stock browser, safari]
- Version [e.g. 22]

**Additional context**
Add any other context about the problem here.