Correct ruby-switch argument when using ruby<VERSION>-full

When setting `ruby::ruby_package => 'ruby1.9.1-full'` (or
`'ruby1.8-full`) the correct argument to `ruby-switch --set` is
`ruby1.9.1` (or `ruby1.8`), not the package name as is. Handle these two
cases explicitly when `ruby::switch` is set. Contains updated spec
tests.