Alias Enumerable#inject as Enumerable#reduce

TODO: Enumerable#inject should accept two arguments, both of which are
optional. Cf. https://github.com/rubyspec/rubyspec/blob/master/core/enumerable/shared/inject.rb
Revert to old Enumerable#inject tests, add a #reduce test
Merge pull request #316 from jstepien/enumerable-reduce

Alias Enumerable#inject as Enumerable#reduce