[DomCrawler] Fixed image input case sensitive

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | 
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

I'm currently testing the payment workflow in a project. The problem is that when I reach the credit card type selection (CB, VISA ...) I cannot submit the desired one because its name is in uppercase (take a look at the screenshot).

![](http://s22.postimg.org/oz5cm3fwh/example.png)