Add missing tests for invalid names in `rails plugin new`

* Test for: Invalid plugin name, because of reserved rails word.
* Test for: Invalid plugin name because of constant name that is already in use.
Allow hyphenated names for `rails plugin new` generator.

It was not possible to create a new gem with a hyphenated name via the `rails plugin new` generator.

The naming guide of rubygems clearly says dashes should be used for gems that extend other gems. http://guides.rubygems.org/name-your-gem/