Make fileInput trigger shiny:inputchanged.

[wch]

Closes #1511. When a fileInput changes, this fires the shiny:inputchanged event.

The format of the data on the client is as close as possible to the format on the server. On the client, it is an array of objects:

[{
  name: "rock.csv",
  size: 1530,
  type: "text/csv"
}]

And on the server, it's a data frame (datapath isn't available on the client):

name      size      type                                    datapath
rock.csv  1530  text/csv  /tmp/RtmpkEIccc/0df954afaddbc88008601d57/0