[WebProfilerBundle] Fixed the form display in the profiler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12949
| License       | MIT
| Doc PR        | ~

With this patch I get the profiler on my form submitted:

![capture d ecran 2014-12-11 a 20 59 16](https://cloud.githubusercontent.com/assets/667519/5401126/0d1be5ca-8179-11e4-8bba-77e232d91d30.png)


same issue here