fix: ensure that VimsImageProcessor#write_to_jpeg writes jpg

[knarewski]

Problem

Vips determines file format using the extension taken from the target filename. That makes VipsImageProcessor#write_to_jpeg's behaviour misleading for names with a different extension (eg image.png) and broken for names with no extension (eg tmp123413123). Example error message:

`write_to_file': VipsForeignSave: "tmp/no-extension" is not a known file format (Vips::Error)

Solution

Write to file with .jpg extension, then rename to target filename

Notes

• code uses the same location as original, so should be performant
• I've also did a little style fix for specs, changed to use context instead of describe where it felt appropriate

[MrLukeSmith]

LGTM