Fix error.stdout|stderr|all type when encoding option is used

[ehmicky]

Right now, the encoding option is not correctly applied to error.stdout|stderr|all. Namely:

• When encoding is buffer, error.stdout|stderr|all is an ArrayBuffer instead of an Uint8Array
• When encoding is something else like hex, error.stdout|stderr|all is a Node.js Buffer instead of a string
• In both cases, our output normalization logic eventually transforms this to undefined

This is because the encoding logic is applied after get-stream has succeeded, but it is not applied on error.bufferedData, i.e. when get-stream has failed or been aborted.

This PR fixes this.

[ehmicky]

error.bufferedData is undefined when the encoding option has an invalid value.