HTTPClientInternalTests.testProxyStreaming fails with unclean shutdown

With #177, there is now a test failure in `HTTPClientInternalTests.testProxyStreaming`:
```
Test Case 'HTTPClientInternalTests.testProxyStreaming' started at 2020-03-05 19:58:19.461
/code/Tests/AsyncHTTPClientTests/HTTPClientInternalTests.swift:110: error: HTTPClientInternalTests.testProxyStreaming : XCTAssertNoThrow failed: threw error "HTTPClientError.uncleanShutdown" - 
```