Always write file encoding comment to File

This Merge Request fixes Bug 426856 by always writing the file encoding comment to the BibTeX file. This causes future instances of KBibTeX to correctly set the file encoding. Without such a comment, KBibTeX assumes the encoding to be UTF-8 and uses that for the next save. Which is fatal when using BibTeX which can't handle unicode well.

I couldn't apply the same to the kbibtex/0.10-branch as it and master seem to be quite diverged.