TextIO.Write: use output directory with custom filenaming scheme

[gene-telligent]

Noticed this while trying to write files (non dynamically) with a custom FileNaming.

Expected behavior: If an output directory is specified with a non-dynamic file write and a custom FileNaming, the generated filename should be resolved against the output directory.

Actual behavior: The output directory is ignored and the result of the FileNaming call becomes the full file path.

No JIRA ticket again because it looks like a quick code fix. @jkff once again can you take a quick look and approve?

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[jkff]

Thank you! Any chance you could add a test for this? I think it should be easy to extract the part that resolves fileNamingFn into a utility function, and add this case as a unit test.

[gene-telligent]

Yep, no problem! Pulled the code into its own function and added some tests for it. Let me know if you have any further feedback!

[jkff]

Thanks! Will dedupe the test code a bit and merge.