Correct RetryProcessor

@Taluu & @ludofleury
It's a bit more complicated as explained in #16.

In fact when you called the `getHeaders` method on an `\AMQPEnveloppe` it returns both properties and headers at the same level (no key "headers" in this array).
BUT, when you published a message with additional headers, you MUST add it in a "headers" key.

What do you think of this implementation ?

I think I have to revert commit 87a956ddccda76be904220c5488a64829e5b4bc6 (`getHeaders` => `getProperties`) to be consistent with libraries implementation, are you agree ?