[DX Enhancement] Make fluent_args works with trailing comma

[juliancoffee]

Currently, this code won't compile:

fluent_args! {
  "name" => "World",
}

Because fluent_args! doesn't handle trailing commas. The fix would be trivial, so I could do a PR if this is appreciated.

[zbraniecki]

I'd appreciate it!

[juliancoffee]

@zbraniecki done!)