def validate_credential_type!
      credentials.each do |credential, value|
        next if value.nil?
        fail(Uber::Error::ConfigurationError.new("Invalid #{credential} specified: #{value.inspect} must be a string or symbol.")) unless value.is_a?(String) || value.is_a?(Symbol)
      end
    end
def validate_credential_type!
      credentials.each do |credential, value|
        next if value.nil?
        fail(Uber::Error::ConfigurationError.new("Invalid #{credential} specified: #{value.inspect} must be a string or symbol.")) unless value.is_a?(String) || value.is_a?(Symbol)
      end
    end
Ensures that all credentials set during configuration are
 of a valid type. Valid types are String and Symbol.

 @raise [Uber::Error::ConfigurationError] Error is raised when
   supplied uber credentials are not a String or Symbol.