Fix #1169 by Proxy-Authorization header generation

[seratch]

Summary

This pull request resolves #1169

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

Merging #1172 (d984b77) into main (0a55e2a) will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1172      +/-   ##
==========================================
+ Coverage   86.58%   86.63%   +0.04%     
==========================================
  Files         110      110              
  Lines       10780    10780              
==========================================
+ Hits         9334     9339       +5     
+ Misses       1446     1441       -5     

Impacted Files	Coverage Δ
slack_sdk/rtm/__init__.py	90.73% <ø> (ø)
slack_sdk/socket_mode/builtin/internals.py	72.44% <50.00%> (+2.22%)	⬆️

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