def credentials(credentials_override = nil)
      credentials = @credentials.dup()
      credentials.merge!(credentials_override) unless credentials_override.nil?
      return credentials
    end
def credentials(credentials_override = nil)
      credentials = @credentials.dup()
      credentials.merge!(credentials_override) unless credentials_override.nil?
      return credentials
    end
Initializes CredentialHandler.
 Returns credentials set for the next call.