Crashes on File Uploads

[sheharyarn]

When the web request contains a %Plug.Upload{} struct, it crashes the process with a Protocol.UndefinedError:

[error] #PID<0.737.0> running MyApp.Endpoint terminated
Server: localhost:5000 (http)
Request: POST /some/server/path
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Enumerable not implemented for %Plug.Upload{content_type: "image/jpeg", filename: "123456789.jpg", path: "/var/folders/rr/1l6bz6rd6tq_h70m1scxy51m0000gn/T//plug-1502/multipart-837302-850377-2"}
        (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
        (elixir) lib/enum.ex:116: Enumerable.reduce/3
        (elixir) lib/enum.ex:1767: Enum.reduce/3
        lib/better_params.ex:79: anonymous fn/2 in BetterParams.symbolize_keys/1
        (stdlib) lists.erl:1263: :lists.foldl/3
        lib/better_params.ex:68: BetterParams.symbolize_merge/1
        lib/better_params.ex:36: BetterParams.call/2
        ...