Add parenthesis around export default assignments (#423)

Fixes #413

src/fast-path.js

@@ -392,6 +392,7 @@ FPp.needsParens = function(assumeExpressionContext) {
         case "LogicalExpression":
         case "LogicalExpression":
         case "NewExpression":
+        case "ExportDefaultDeclaration":
           return true;
 
         case "CallExpression":

tests/export_default/__snapshots__/jsfmt.spec.js.snap

@@ -5,6 +5,13 @@ module.exports = require(\"M\");
 "
 `;
 
+exports[`test body.js 1`] = `
+"export default (class {}[1] = 1);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+export default (class {}[1] = 1);
+"
+`;
+
 exports[`test test.js 1`] = `
 "var M = require(\'M\');
 var N = require(\'N\');

tests/export_default/body.js

@@ -0,0 +1 @@
+export default (class {}[1] = 1);