def class_for(provider_name)
      provider_name = provider_name.to_sym

      providers.fetch(provider_name)
    rescue KeyError
      raise ProxyFetcher::Exceptions::UnknownProvider, provider_name
    end
def class_for(provider_name)
      provider_name = provider_name.to_sym

      providers.fetch(provider_name)
    rescue KeyError
      raise ProxyFetcher::Exceptions::UnknownProvider, provider_name
    end
Returns a class for specific provider if it is registered
 in the registry. Otherwise throws an exception.

 @param provider_name [String, Symbol]
   provider name

 @return [Class]
   provider class

 @raise [ProxyFetcher::Exceptions::UnknownProvider]
   provider is unknown