Downloading PDFs over https intermittently fails

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=6118

Reported by Erik Allemann erik@roxen.com

RT#17711
Tested on 5.1-rel1

Downloading a (semi-)large PDF over https occasionally fails.

Symptoms vary:

1. The browser sometimes visually loads the PDF, but seemingly never completes the download.
2. The browser indicates complete file download but never displays the PDF content (the browser may still show as loading the page/file).
3. Occasionally the PDF browser plugin looks like its reporting an incorrect file size.

Debug example is from #2 (attached) as well as example PDF.