Add missing configuration key

[greg0ire]

The code and the error messages say it should be like this.

[Nyholm]

Are you sure about this?

[greg0ire]

I could be mistaken, I'm not configuring the bundle directly. But what do you think about this : https://github.com/php-http/HttplugBundle/blob/master/DependencyInjection/Configuration.php#L308 ? I can agree that this key looks a bit useless though.

[dbu]

the configuration indeed seems to request this format.

i think the key is useful, if we ever would add anything else than the list of headers, we would need a BC break if we don't wrap the list under its own key.

[greg0ire]

@dbu makes sense