Support for defaults when destructuring parameters

[tswaters]

The following code:

({a = 'b'}) => {}

When run through esprima returns:

{
    "type": "Program",
    "body": [
        {
            "type": "ExpressionStatement",
            "expression": {
                "type": "ArrowFunctionExpression",
                "id": null,
                "params": [
                    {
                        "type": "ObjectPattern",
                        "properties": [
                            {
                                "type": "Property",
                                "key": {
                                    "type": "Identifier",
                                    "name": "a"
                                },
                                "computed": false,
                                "value": {
                                    "type": "AssignmentPattern",
                                    "left": {
                                        "type": "Identifier",
                                        "name": "a"
                                    },
                                    "right": {
                                        "type": "Literal",
                                        "value": "b",
                                        "raw": "'b'"
                                    }
                                },
                                "kind": "init",
                                "method": false,
                                "shorthand": true
                            }
                        ]
                    }
                ],
                "defaults": [],
                "body": {
                    "type": "BlockStatement",
                    "body": []
                },
                "generator": false,
                "expression": false
            }
        }
    ],
    "sourceType": "script"
}

And when run back through escodegen, returns:

({a}) => {};

i.e., the value of a is not recognized and the AssignmentExpression is lost

..

I was playing around with this, and I found that adding expr.value as a third parameter to generatePropertyKey and modifying like so makes it work

-        result.push(this.generateExpression(expr, Precedence.Sequence, E_TTT));
+        if (value.type === 'AssignmentPattern') {
+            result.push(this.AssignmentPattern(value, Precedence.Sequence, E_TTT));
+        }
+        else {
+            result.push(this.generateExpression(expr, Precedence.Sequence, E_TTT));
+        }

Thing is, I'm unsure if this could have other unintended consequences...the tests still pass, fwiw.

I can send along a PR with this change, and a simple object destructuring test... thoughts?