Parsing Inconsistency with Multiple Values for an HTTP Header

For an `HttpTrigger` function, the provided `HttpRequestMessage` and its `HttpHeaders` collection has different behavior with respect to duplicate headers when executing locally and when executing in Azure. The behavior when executing in Azure appears to be incorrect.

#### Investigative information

Azure Functions Core Tools (2.7.1846 Commit hash: 58c671341fda1c52bd46e1aa8943cb26e467830)
Function Runtime Version: 2.0.12858.0
.NET Core SDK: 3.0.101

#### Repro steps

Suppose the request sent to a function has additional HTTP headers specified such as:

```
X-Some-Header: ValueA
X-Some-Header: ValueB
```

#### Expected behavior

I expect the `HttpHeaders` collection to have an entry with a key of `"X-Some-Header"` and an enumerable value consisting of two separate strings such as: `["ValueA", "ValueB"]`. This is the behavior observed when running the function locally.

#### Actual behavior

When running the function in Azure, the `HttpHeaders` collection has an entry with a key of `"X-Some-Header"` and an enumerable value consisting of a single concatenated string like: `["ValueA, ValueB"]`.

#### Known workarounds

Obviously one could split the header values, but this may be unreliable if the header values can contain a comma.

This subtle difference in behavior between environments was not expected and could be difficult to identify and troubleshoot.

Start with the HttpTrigger handling of HttpRequestMessage and its HttpHeaders collection, reproducing the duplicate X-Some-Header request locally and in Azure using the versions listed. Trace where duplicate values are converted between the two environments; done means Azure exposes ["ValueA", "ValueB"] rather than ["ValueA, ValueB"] without breaking comma-containing values.

Written by the indexing model from the issue text.