explicit default of 'UTF-8' for kernel_api()

[wibeasley]

somewhat related to #269, I'll add an encoding parameter for kernel_api() that defaults to "UTF-8". I doubt it will be used much in this state, but it will help debugging issues like #269. If it proves useful, the encoding could be a parameter of callers (like redcap_read()) that pass the value to kernel_api().