#6414-Add a condition for captcha element

I added a condition when the element is a Captcha. This way, no more validator errors. Just reCaptcha trigger an error validation about the usage of frameborder attribute in iframe tag, but it is reCaptcha related. Not zf2 responsibility.

I didn't write any tests, given the fact it is related to html validation. But I run the usual tests, and nothing goes wrong.

I hope I didn't messed up everything, since it is my first PR.

Tell me if something wrong.

Thanks.
Julien.