allow writeYaml and writeJSON to return text without requiring a node

[rittneje]

Allow writeYaml and writeJSON to run with returnText: true without requiring a node. This is the same way that readYaml and readJSON work when using the text parameter.

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[rittneje]

@rsandell This is effectively a bugfix for my other two PRs (#88 and #90). I didn't realize this also needed to be changed.