apt https transport not encoding '+', breaks S3

[Phil Pennock]

Encountered with Trusty, apt package 1.0.1ubuntu2

When using a repository in `/etc/apt/sources.list.d` which is `https` pointing to a site provided by Amazon's S3 storage system, `apt-get` will fail with a "403 Forbidden" error message if the package name has a "+" in it.

Reproducible with curl -- S3 does not like unencoded "+" signs, on either HTTP or HTTPS.

`apt-get` works against S3 with HTTP but does not work with HTTPS, which strongly suggests a difference in how package names are encoded for the transport request.