Support query string binding for Option<'T>

[diegobfernandez]

When binding from a query string to a record with a field of type int option this exception happens.
Haven't try with form/json/xml binding.

System.AggregateException: One or more errors occurred. ---> System.NotSupportedException: TypeConverter cannot convert from System.String.
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at Giraffe.HttpContextExtensions.HttpContext-BindQueryString@86-1.Invoke(PropertyInfo p)
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at Giraffe.HttpContextExtensions.HttpContext-BindQueryString@82.Invoke(Unit unitVar)
   at Microsoft.FSharp.Control.AsyncBuilderImpl.callA@841.Invoke(AsyncParams`1 args)
   --- End of inner exception stack trace ---
---> (Inner Exception #0) System.NotSupportedException: TypeConverter cannot convert from System.String.
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at Giraffe.HttpContextExtensions.HttpContext-BindQueryString@86-1.Invoke(PropertyInfo p)
   at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source)
   at Giraffe.HttpContextExtensions.HttpContext-BindQueryString@82.Invoke(Unit unitVar)
   at Microsoft.FSharp.Control.AsyncBuilderImpl.callA@841.Invoke(AsyncParams`1 args)<---

[nojaf]

This is actually a Json.NET thing, see https://gist.github.com/isaacabraham/ba679f285bfd15d2f53e.
Might be interesting to include these extra converters by default in Giraffe.
@dustinmoris thoughts?

[diegobfernandez]

I meant Query String binding :)
This is the model I'm trying to bind to:

[<CLIMutable>]
type Query =
    { pattern: string
      page: int
      perPage: int
      userId: string
      dataViewId: int option }

and the code to do the binding:

let! query = ctx.BindQueryString<Query>()

That line gives me the exception.
I can give it a try and submit a PR if it seems doable.

I don't currently use JSON payload on my requests, althoug that link provide a lot of insights for future needs, I think for now it should be mentioned on docs and after people feedback may be enabled by default.

[nojaf]

Oh, my bad.

[dustinmoris]

Hey guys, yes I think we could add a case for options to the binding of the query string.

[diegobfernandez]

I'll do a PR for this soon, just finishing the tests.