Add support for resuming downloads with Firefox

[Nicklas Nordborg]

Firefox has a feature when downloading files that allows a user to pause a download and then resume it later. However, this feature doesn't work when downloading files from BASE. Firefox displays an error:

<filename> could not be saved, because the source file could not be read.

Try again later, or contact the server administrator.

The main reason behind the problem is that when resuming, Firefox submits a second download request to BASE that includes a "Range" header that include information about how much of the file that has already been downloaded.