Populate responseText when empty for jQuery 1.10

For some reason, when I updated to jQuery 1.10, the `jqXHR.responseText` stopped being populated with the text response value from the ajax request. I have no idea why this is, and I've verified that it only happens with jQuery 1.10.0 through 1.10.2.

Basically I have a request being submitted with `iframe: true`, and I have a success callback attached, which looks like this:

```js
function(evt, data, status, xhr) { console.log('success!', xhr); $('#comments').after(xhr.responseText); });
```

So, it's logging the `xhr` object to the console and then appending the `responseText` value to the DOM (I'm dealing with `dataType: 'html'` here). With jQuery 1.8 or 1.9, it works as expected, and you get this in the console:

```js
success! 
Object {readyState: 4, getResponseHeader: function, getAllResponseHeaders: function, setRequestHeader: function, overrideMimeType: function…}
abort: function ( statusText ) {
always: function () {
complete: function () {
done: function () {
error: function () {
fail: function () {
getAllResponseHeaders: function () {
getResponseHeader: function ( key ) {
overrideMimeType: function ( type ) {
pipe: function ( /* fnDone, fnFail, fnProgress */ ) {
progress: function () {
promise: function ( obj ) {
readyState: 4
responseText: "<span>↵HTML response:↵</span>↵↵<table>↵<tr class='comment' id='comment-365'>↵  <td>asdf</td>↵  <td>asdf</td>↵  <td><a href="/system/attachments/365/original/humans.txt?1373052896">humans.txt</a></td>↵  <td></td>↵  <td><a href="/comments/365">Show</a></td>↵  <td><a href="/comments/365/edit">Edit</a></td>↵  <td><a href="/comments/365" data-confirm="Are you sure?" data-method="delete" data-remote="true" rel="nofollow">Destroy</a></td>↵</tr>↵↵</table>↵"
setRequestHeader: function ( name, value ) {
state: function () {
status: "200"
statusCode: function ( map ) {
statusText: "OK"
success: function () {
then: function ( /* fnDone, fnFail, fnProgress */ ) {
__proto__: Object
```

Notice the `responseText` populated above. When updating to jQuery 1.10.x, nothing gets appended to the DOM, due to the fact that `responseText` is blank. And you get this in the console:

```js
success! 
Object {readyState: 4, getResponseHeader: function, getAllResponseHeaders: function, setRequestHeader: function, overrideMimeType: function…}
abort: function ( statusText ) {
always: function () {
complete: function () {
done: function () {
error: function () {
fail: function () {
getAllResponseHeaders: function () {
getResponseHeader: function ( key ) {
overrideMimeType: function ( type ) {
pipe: function ( /* fnDone, fnFail, fnProgress */ ) {
progress: function () {
promise: function ( obj ) {
readyState: 4
setRequestHeader: function ( name, value ) {
state: function () {
status: "200"
statusCode: function ( map ) {
statusText: "OK"
success: function () {
then: function ( /* fnDone, fnFail, fnProgress */ ) {
__proto__: Object
```

Notice that `responseText` is missing. I did some debugging, originally thinking maybe jQuery changed the order of events firing and that the success callback was getting called before jQuery populated it, but after further debugging it seems jQuery just never populates it when using the transport when a non-standard `dataType` is specified.

So, to summarize, this only happens when all three of the following conditions are true:

1. Using this iframe-transport.
2. Using jQuery 1.10.x.
3. The `dataType` is specified (if it's blank, and the defaults are used, it works fine).

Adding this to iframe-transport fixed it and seems to be backwards compatible with jQuery 1.9, etc. as well.