Fix extra newline append for multipart file write

[dceejay]

ref Issue #3913

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

If you write a file (previously read in - with msg.parts attached) - in per line mode - with append linefeed set - we currently add an extra line feed due to the blank / null line at the end that closes the file. This fix detects that and stops it happening so you can write out an identical file to that read in more correctly..

This is to address the Issue #3913

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[dceejay]

Test fail in http-request node for no apparent reason... maybe will pass once latest dev is merged ?

[knolleary]

Merging is blocked until #3922 is merged and this branch rebased to pickup that fix.

[coveralls]

Coverage increased (+0.008%) to 68.294% when pulling 95a7980 on Fix-file-write-last-line-newline-append into 14e74af on master.