Fixed problem with submitting form by input[type=image]

I couldn't submit form by input[type=image], using css or xpath.

html:

```
<input type="image" src="button.gif" alt="Submit" />
```

test:

```
OK: $I->click("Submit");
NG: $I->click("input[type=image]")
NG: $I->click("input[@type='image']")
```