ClosedXML Extensions WebAPI does not work with ASP .NET Core Web API

Following up from @Pankraty's [comment on ClosedXML/#1283](https://github.com/ClosedXML/ClosedXML/issues/1283#issuecomment-526469962)... Looking at the different ClosedXML extensions, it seems [`ClosedXML.Extensions.AspNet`](https://github.com/ClosedXML/ClosedXML.Extensions.AspNet) would be the most appropriate extension for ASP .NET Core Web API projects, so the below is just for awareness.

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Instead of the file being downloaded, we receive a text response:


Sample project that reproduces the above:
* [TestClosedXMLWebApi.zip](https://github.com/ClosedXML/ClosedXML.Extensions.WebApi/files/3560345/TestClosedXMLWebApi.zip)

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NuGet packages added to the project:

```xml

```

Example code from the [README](https://github.com/ClosedXML/ClosedXML.Extensions.WebApi#usage):

```csharp
[ApiController]
public class ExcelController : ControllerBase
{
[HttpGet]
[Route("api/file/{id}")]
public async Task DownloadFile(int id)
{
var wb = await BuildExcelFile(id);
return wb.Deliver("excelfile.xlsx");
}

private async Task BuildExcelFile(int id)
{
var t = Task.Run(() =>
{
var wb = new XLWorkbook();
var ws = wb.AddWorksheet("Sheet1");
ws.FirstCell().SetValue(id);

return wb;
});

return await t;
}
}
```