Fix ForwardCompatAuthenticatorTrait with OPCache preload

[elavrom]

Hi there,

Since the eval() in the ForwardCompatAuthenticatorTrait is creating problems when OPCache preload is enabled, here is a quick fix to avoid eval()'ing multiple times.

(Issue ref : #996)

P.S. : First time making a PR, feel free to tell me if I did something wrong

[Amegatron]

I still wonder, why there is need for eval. It seems to be OK to have just normal Trait declarations. Instead, eval indeed may cause problems with different kinds of static analysis and caching.

[chalasr]

The reason is that the v2 of this package supports a wide range of PHP versions including legacy ones that do not support some type declarations. Using eval is the only way to make php skip interpreting it and avoid ending up with a fatal error.

[chalasr]

Thank you @elavrom.

[Amegatron]

@chalasr Thanks for clarification. I believe this reason now especially highlights what I described in the mentioned issue: #996 (comment)

In brief, my belief is that another compatibility approach should be chosen.