Adding host details in proxy headers

[HarikrishnanTechie]

Issue:
The connect request does not include host details when operating in a proxy environment. This can lead to failures in establishing a proper connection, as the host information is essential for routing the request through the proxy.

Code Explanation:
To address this issue, the following approach is used:

Check for Host in Proxy Settings:
If the proxy_kwargs dictionary contains a host key, it indicates that host details are available.

Add Host to Proxy Headers:
These host details are added to the proxy_headers dictionary under the Host key. By doing so, the host information is explicitly included in the proxy headers, ensuring that the connect request incorporates the correct routing details.

Impact:
Including the host details in the proxy_headers ensures that the connect request is properly formed and able to route through the proxy environment without any missing information. This resolves the issue and enhances connectivity reliability

[sigmavirus24]

We should resolve urllib3/urllib3#3532 first. Both of these PRs lack tests and the correct way to handle the header in question