Issue with ExportSpecifier

[adamjmcgrath]

I've noticed an issue in 1.7.1 with ExportSpecfier caused by b99a200

var esprima = require('esprima');
var escodegen = require('escodegen'); // 1.7.1

escodegen.generate(esprima.parse('export { foo };', sourceType: 'module'));

TypeError: Cannot read property 'name' of undefined
    at CodeGenerator.Expression.ExportSpecifier (escodegen/escodegen.js:2266:54)
    at CodeGenerator.generateExpression (escodegen/escodegen.js:2428:28)
   ...

On this line:
https://github.com/estools/escodegen/blob/master/escodegen.js#L2265
var exported = (expr.id || expr.imported).name;

This should either include the ExportSpecifier:
https://github.com/estree/estree/blob/master/es6.md#exportspecifier
var exported = (expr.id || expr.imported || expr.exported).name;

Or remove the ImportSpecifier and just include the ModuleSpecifier
https://github.com/estree/estree/blob/master/es6.md#modulespecifier
var exported = (expr.id || expr.local).name;

I'll raise a PR with a fix and some test cases to prevent this regression

[michaelficarra]

Fixed by #272.