Azure Functions v3 HttpTrigger: request body is empty

HttpTrigger works incorrectly in the following conditions:
* Azure Functions v3
* TargetFramework: netcoreapp3.1
* `Microsoft.NET.Sdk.Functions` version `3.0.2`
* Code:
```
[FunctionName("RunNetCore3")]
public static async Task RunNetCore3Async(
[HttpTrigger(AuthorizationLevel.Anonymous, "POST", Route = "netcore3/{id}")]
CustomRouteParameter parameter,
HttpRequest request,
CancellationToken cancellationToken)
{
using (var reader = new StreamReader(request.Body, Encoding.UTF8))
{
return await reader.ReadToEndAsync();
}
}
```
* The request body in the `request` variable is empty if request contains `Content-Type: application/json` header (works ok with any other `Content-Type` values except `application/json`). So in the example the empty response body will be returned which is unexpected.

#### Investigative information

I've created the [repository](https://github.com/myarotskaya/AzureFunctions.NetCore3.Bug) to demonstrate this issue. The source code of this repository is deployed to the Function App.

- Timestamp: 09:27 AM (UTC Time)
- Function App version: 3.0
- Function App name: NetCore3HttpBindingBug
- Function name(s) (as appropriate): RunNetCore3
- Invocation ID: `32894c31-74ac-4a98-8f94-fe6c76662d7d`
- Region: Central US

#### Repro steps

* Send `POST https://netcore3httpbindingbug.azurewebsites.net/api/netcore3/qqq` request with some body and `Content-Type: application/json` header.

#### Expected behavior

Request's body is returned in the response.

#### Actual behavior

Request's body is not returned in the response.

#### Known workarounds

Do not send `Content-Type: application/json` header or send any other value in the `Content-Type` header.

#### Related information

The same code works correct in Azure Functions v2 on netcoreapp2.1 with `Microsoft.NET.Sdk.Functions` version `1.0.29` and in ASP.NET Core 3.1 Web Application.

There are two endpoints to check it (see the [repository](https://github.com/myarotskaya/AzureFunctions.NetCore3.Bug)):
* Send `POST /netcore2/some-id` request with some body and `Content-Type: application/json` header
* Send `POST /aspnetcore3/some-id` request with some body and `Content-Type: application/json` header

In both cases the request body will be returned in the response (I didn't deploy it to any Function App but it's reproducible locally as well).