Fix for issue #156

Added a utility function to parse a string to a DateTime option, and used this to check for invalid headers.

I have no idea how to write a test for this, although I suspect we probably should.
lastly here this is an error, `send_it` it is used to send the resource to the client. This branch should be just

```fsharp
| Choice2Of2 parseError ->
   BAD_REQUEST ctx
```