gsub  breaks \b for checking word boundaries

When I run the following program with `jq -n`:

```
"hello world"|"\\b(?<i>[[:lower:]])" as $rx|gsub($rx;.i|ascii_upcase)
```

Expected Output:

> `"Hello World"`

Actual Output:

> `"HELLO WORLD"`

I've got a workaround for my particular use case:

```
"hello world"|"\\b(?<fst>[[:lower:]])(?<rst>[[:lower:]]*)\\b" as $rx|gsub($rx;"\(.fst|ascii_upcase)\(.rst)")
```