HTTP routes don't work for optional strings

[brettsam]

From this post: http://stackoverflow.com/questions/43748806/how-to-do-rest-api-optional-string-uri-parameter-with-azure-function-http-bindin

You can't add a route like "route": "books/{bookName?}" and use a string for bookName in your function -- we'll throw if the bookName part of the URL is missing.

The issue looks to be here: https://github.com/Azure/azure-webjobs-sdk/blob/dev/src/Microsoft.Azure.WebJobs.Host/Bindings/Data/ClassDataBinding.cs#L63. If the value of the object is null, that evaluates to false, and you get the exception.

This works for ints (for example) because they are structs and that is handled differently here: https://github.com/Azure/azure-webjobs-sdk/blob/dev/src/Microsoft.Azure.WebJobs.Host/Bindings/Data/StructDataBinding.cs#L71