ApiFormatBase: Encode filenames in Content-Disposition

ApiFormatBase: Encode filenames in Content-Disposition

The return value for ApiFormatBase::getFilename() was formerly
documented as "must be encoded for inclusion in a Content-Disposition
header's filename parameter." While this is ok for the common use case
where the module is returning a constant string or can assume whatever
it gets back from getExtensionsForType() is ok, it's not in general a
good idea to make all callers handle that.

Further, it's not possible to represent characters outside of the
ISO-8859-1 character set in a 'filename' parameter. You have to use
'filename*' to do that (see RFC 5987 and RFC 6266).

So, this patch changes the definition of getFilename() to remove the
encoding requirement, and adds code to properly convert and escape the
value for the 'filename' and (if necessary) 'filename*' parameters.

Note this may give unexpected results (double encoding) if any module
actually is returning an encoded filename. I don't see any such cases in
core or in extensions in Gerrit.

Change-Id: I0c2749a847b639f228efff4e1917a61612a1f7d1