User-Agent correctness issues

[kornelski]

Sparkle's default user-agent contains app's name and version.

• the app name can be non-ASCII (e.g. CFBundleDisplayName localized in Japanese). Cocoa replaces non-ASCII characters with ?, resulting in UA like ???????/1.0 Sparkle/1.15.

• the app version is taken from displayVersion, which isn't the exact unique version of the build.

Those are small issues that affect collection of update statistics.