Handle filenames with spaces

• This breaks all tests, because expected filenames no longer match the ones that are actually written. I would appreciate your input on how to fix this, or what should be done about it.
• There was originally a test added in this PR for the behavior, but it failed even after the change was made (and not for the quoting reason.) I don't know what was wrong with it, but a better test should be written instead.

For my notes while developing this, see my comments in #36.

该提问来源于开源项目：tectonic-typesetting/tectonic