add space before the header value in getAllResponseHeaders

[ewfian]

This pull request add a single space before header value (after colon). then the polyfilled getAllResponseHeaders() will return the same format value with native

1. The origin http header
   

2. Parsed response header
   

3. The return value of polyfilled getAllResponseHeaders()
   The space lost here
   

4. The FIRST CHAR LOST.
   Some 3rd libraries like azure-sdk-for-js makes http request by using XMLHttpRequest and parses header by itself just like following code
   https://github.com/Azure/azure-sdk-for-js/blob/744e51acb755db2c565c71a8856d8d13e6ec345b/sdk/core/core-https/src/xhrHttpsClient.ts#L129
   If the return value from the polyfilled function XMLHttpRequest.getAllResponseHeaders() is not keeps the exact same format (Date: Thu, 01 Oct 2020 05:18:27 GMT\r\n) as the original value, some thing will broken.
   

[ewfian]

Sorry for contributing rules. I will close this pull request and create an issue