Fix invalid pluralization of Spree::RelationType

[jumski]

I believe this is same stuff as #107 (related to spree/spree#5842)

Please tell if i should base this on master.

Thanks

[futhr]

Hi @jumski, this fix works for both 3-0-stable and master.

[jumski]

So this should be based on master? Not sure how you propagate this to the stable branch.

[futhr]

Hi @jumski, no worries we can cherry-pick it across branches when the failing specs been resolved.

[jumski]

I don't know how to fix this error, seems unrelated to my changes.

I have checked out 3-0-stable branch and running tests results in same error:

/home/jumski/.rvm/gems/ruby-2.1.5/bundler/gems/spree-438dd5e58c0d/core/lib/spree/testing_support/controller_requests.rb:32:inblock in module:ControllerRequests': undefined method routes' for #<Class:0x00000007181698> (NoMethodError)