admin/functions/download does not respect ARM max response size limits

#### Investigative information
The following method in `FunctionsController.cs` https://github.com/Azure/azure-functions-host/blob/7054c51ad9666d6ae0f30da57054d9d8c457789b/src/WebJobs.Script.WebHost/Controllers/FunctionsController.cs#L221 does not seem to respect ARM maximum response size limits, documented here: https://github.com/Azure/azure-resource-manager-rpc/blob/master/v1.0/common-api-details.md#max-response-size.

#### Expected behavior

Calling the endpoint provides a response within the maximum response size limit of 8MB.

#### Actual behavior

Downloads appear to be able to exceed that limit.

Start in src/WebJobs.Script.WebHost/Controllers/FunctionsController.cs at the FunctionsController method linked in the issue, then review the ARM maximum response-size documentation. Verify the admin/functions/download behavior with a response over 8 MB; done means the endpoint stays within the documented 8 MB limit.

Written by the indexing model from the issue text.