Uncurried mode: error message claims incorrect required number of args in function with optional args

Repro:
```rescript
let f = (~a=0, b, c) => a + b + c
let x = f(42)
```
yields:
```
This uncurried function has type (~a: int=?, int, int) => int
  It is applied with 1 arguments but it requires 3.
```

There's only 2 required arguments in this function. The third is optional.