Test Failure: System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success

## Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579855
Build error leg or test failing: System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success
```
System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success(numHeaders: 1000) [FAIL]
System.ArgumentNullException : Value cannot be null. (Parameter 'request')
```

## Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorPattern": "System\\.Net\\.Tests\\.SimpleHttpTests\\.UnknownHeaders_Success.+\\[FAIL\\]",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Report

|Build|Repository|Test|Pull Request|
|---|---|---|---|
|[1584906](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584906)|dotnet/runtime|[System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584906&view=ms.vss-test-web.build-test-results-tab&runId=43731396&resultId=138245)||
|[1584422](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584422)|dotnet/runtime|[System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584422&view=ms.vss-test-web.build-test-results-tab&runId=43716152&resultId=154477)||
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|2|

Start with System.Net.Tests.SimpleHttpTests.UnknownHeaders_Success and the linked Azure build/test results; reproduce the failure for numHeaders: 1000 and inspect the request-null exception. Done means the test passes reliably and the underlying failure is covered by the test suite.

Written by the indexing model from the issue text.