Fixed Column Accessor bug

example/src/screens/index.js

@@ -21,7 +21,8 @@ export default Component({
         accessor: 'firstName'
       }, {
         header: 'Last Name',
-        accessor: 'lastName'
+        id: 'lastName',
+        accessor: d => d.lastName
       }]
     }, {
       header: 'Info',

src/index.js

@@ -126,12 +126,14 @@ export default React.createClass({
         return dcol
       }
 
-      if (dcol.accessor && !dcol.id) {
+      if (!dcol.accessor) {
+        dcol.accessor = d => undefined
+      }
+      if (!dcol.id) {
         console.warn(dcol)
         throw new Error('A column id is required if using a non-string accessor for column above.')
       }
 
-      dcol.accessor = d => undefined
       return dcol
     }