Change setProvider() method to use a class constant when specifying providers.

[allebb]

A class constant should be used instead for best practice, ease of use and potentially less prone to typo mistakes etc.

This will not break any existing code as the plaintext will work fine too but I personally feel that being able to use a class constant is a more neater solution.

[allebb]

Stupidly I forgot that the provider classes utilise the ProviderInterface and are looked up at runtime, therefore it would be silly to manage a static array of provider names given that my initial implementation was to have it dynamic.