Fix output arrow function with 1 param with default value

Fixes #1090

test/compress/arrow.js

@@ -82,4 +82,11 @@ arrow_functions_with_body: {
             b.join();
         };
     }
-}
+}
+
+arrow_function_with_single_parameter_with_default: {
+    input: {
+        var foo = (a = 0) => doSomething(a);
+    }
+    expect_exact: "var foo=(a=0)=>doSomething(a);"
+}