Parse error using acorn, even though standalone acorn can handle it

Using some new syntax, like object spread but also jsx, recast throws an error while parsing. This is even though I supply acorn as a parser and acorn can handle it on its own.

I have 2 code examples:
**spread:**
```
const a = { foo: 'bar' }
const b = { baz: 'qux' }

const merged = { ...a, ...b }
```

**jsx**
```

Both of these can be parsed if I do:
```
const acorn = require('acorn')
const jsx = require('acorn-jsx')
const acornParser = acorn.Parser.extend(jsx())
acornParser.parse(code)
```

But they fail if I add
```
recast.parse(code, { parser: acornParser })
```
With the alternative setup, _spread_ works but _jsx_ fails again
```
recast.parse(code, {
parser: {
parse (codeString) {
const ast = acornParser.parse(codeString)
return ast
}
}
})
```

I have created a repository to demo this. Clone and run `npm start` to reproduce. https://github.com/TheLudd/recast-bug

The error messages I get are:

**Spread:** `SyntaxError: Unexpected token (4:17)`

**jsx:** `Error: Line 1: Invalid regular expression: missing /`

Clone the linked demo repository and run `npm start` to reproduce both failures. Compare direct `acornParser.parse(code)` with the two `recast.parse` configurations shown in the issue, using the spread and JSX examples. Done means both examples parse through recast without the reported errors.

Written by the indexing model from the issue text.