Header value null check

[alexeyzimarev]

Description

Ensure that when adding a header parameter with null value, it throws immediately, without waiting for request execution.
Allow multiple header values for one header key, also support properly adding header parameters with multiple values per header.

Fixes#2227
Fixes#2235

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

[github-actions]

Test Results

35 files 35 suites 19m 56s ⏱️
438 tests 438 ✅ 0 💤 0 ❌
2 965 runs 2 965 ✅ 0 💤 0 ❌

Results for commit df05ab9.

♻️ This comment has been updated with latest results.

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