Allow setting parts for http trigger response when function returns a json

cc @mhoeger

Following code allows setting StatusCode, Headers, Query etc on http response only if body is set.

https://github.com/Azure/azure-functions-host/blob/680243f51bae49dc176142f72378c707665dce71/src/WebJobs.Script/Binding/Http/HttpBinding.cs#L118-L142

This code effects out-of-proc and .csx functions. Fixing it would be breaking change. Have to fix this as part of next major release.

Start by reviewing src/WebJobs.Script/Binding/Http/HttpBinding.cs at lines 118-142, where HTTP response properties are currently handled. Compare behavior for out-of-proc and .csx functions when the function returns JSON, and account for the stated breaking-change constraint. Done means those responses can set StatusCode, Headers, Query, and related properties without affecting existing compatibility expectations.

Written by the indexing model from the issue text.