AzureFunctions Proxy x-ms-privatelink-id causing invalid header value check

When function is routed through Azure Functions Proxy, a x-ms-privatelink-id header value is added.

This header value has invalid characters

"x-ms-privatelink-id":"\n\nrequest-id\u00123|50e85f514b894e43945890308a5081ab.029e71f938874c4d."

Which fails "fetch-node" 's Headers validation check.

#### Investigative information

Please provide the following:

- Timestamp:
- Function App version:
- Function App name:
- Function name(s) (as appropriate):
- Invocation ID:
- Region:

#### Repro steps

Provide the steps required to reproduce the problem:

````
import { AzureFunction, Context, HttpRequest } from "@azure/functions"
import { Headers } from "node-fetch"

const httpTrigger: AzureFunction = async function (context: Context, req: HttpRequest): Promise {
context.log('HTTP trigger function processed a request.');

context.log( JSON.stringify(req.headers) );

let h = new Headers(req.headers);

context.log( JSON.stringify(h) );

const name = (req.query.name || (req.body && req.body.name));
const responseMessage = name
? "Hello, " + name + ". This HTTP triggered function executed successfully."
: "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response.";

context.res = {
// status: 200, /* Defaults to 200 */
body: responseMessage
};
};

export default httpTrigger;
````

#### Expected behavior

Provide a description of the expected behavior.

When function is called directly - no issues.
When function is called through an AzureFunctions proxy - no issues.

#### Actual behavior

Provide a description of the actual behavior observed.

When function is called through an AzureFunctions proxy - fails with

2020-05-07T08:25:32.807 [Information] {"accept":"*/*","accept-encoding":"gzip","cache-control":"no-cache","connection":"Keep-Alive","content-length":"0",
...
,"x-ms-privatelink-id":"\n\nrequest-id\u00123|50e85f514b894e43945890308a5081ab.029e71f938874c4d.","traceparent":"00-50e85f514b894e43945890308a5081ab-029e71f938874c4d-00","x-arr-ssl":"2048|256|C=US, S=Washington, L=Redmond, O=Microsoft Corporation, OU=Microsoft IT, CN=Microsoft IT TLS CA 5|CN=*.azurewebsites.net","x-appservice-proto":"https"}
2020-05-07T08:25:33.293 [Error] Executed 'Functions.testRedirect' (Failed, Id=a704c0e8-ad73-4878-89ac-df4e67de962d)
Result: Failure
Exception: TypeError:

request-id3|50e85f514b894e43945890308a5081ab.029e71f938874c4d. is not a legal HTTP header value
Stack: TypeError:

request-id3|50e85f514b894e43945890308a5081ab.029e71f938874c4d. is not a legal HTTP header value
at validateValue (D:\home\site\wwwroot\node_modules\node-fetch\lib\index.js:677:9)
at Headers.append (D:\home\site\wwwroot\node_modules\node-fetch\lib\index.js:829:3)
at new Headers (D:\home\site\wwwroot\node_modules\node-fetch\lib\index.js:754:11)
at Object. (D:\home\site\wwwroot\dist\testRedirect\index.js:17:17)
at Generator.next ()
at D:\home\site\wwwroot\dist\testRedirect\index.js:8:71
at new Promise ()
at __awaiter (D:\home\site\wwwroot\dist\testRedirect\index.js:4:12)
at Object.httpTrigger [as default] (D:\home\site\wwwroot\dist\testRedirect\index.js:14:12)
at D:\Program Files (x86)\SiteExtensions\Functions\2.0.13351\workers\node\worker-bundle.js:18808:26

#### Known workarounds

Provide a description of any known workarounds.

Don't use Node-Fetch? But this is a depedency of ApolloServer for AzureFunctions that I can't remove.

#### Related information

Provide any related information

This appears to be a most recent bug that started happening on 2020-05-06

Tried to remove x-ms-privatelink-id in AzureFunctions Proxy config, but it comes back regardless.

Start with the Azure Functions Proxy path and the sample testRedirect entry point, then compare the logged headers with node_modules/node-fetch/lib/index.js where Headers validation fails. Done means the proxied request no longer causes the invalid x-ms-privatelink-id value to fail header construction, with the behavior verified against the supplied reproduction.

Written by the indexing model from the issue text.