Fix regression in generator around belongs_to options compatibility

[lorefnon]

Fixes #392

As mentioned by @NerdDiffer in a417a42#commitcomment-16775070 usage of :optional => true introduces an ArgumentError in Rails < 5.

This was not caught in our specs, because the generator specs do not actually execute the generated files. While I intend to improve the generator specs to handle such errors in a future PR, this one attends to the regression at hand.

[wldcordeiro]

Thanks for this @lorefnon I'll be merging this and cutting a 5.1.1 tomorrow. Ping me if you're interested in joining the contributors and I'll add you to the community.

[coveralls]

Coverage increased (+8.8%) to 92.094% when pulling c5642cc on lorefnon:bugfix/392 into 2a61bcb on RolifyCommunity:master.

[coveralls]

Coverage increased (+8.8%) to 92.094% when pulling c5642cc on lorefnon:bugfix/392 into 2a61bcb on RolifyCommunity:master.

[coveralls]

Coverage increased (+10.3%) to 93.552% when pulling 731b222 on lorefnon:bugfix/392 into 2a61bcb on RolifyCommunity:master.

[lorefnon]

@wldcordeiro I will be happy to join the community. Though I see the library as fairly feature complete as it is, I would continue to help with feature requests, enhancements and regressions as and when time permits.

[lorefnon]

@wldcordeiro I believe we should get another release out as multiple people have faced this issue.

While I can merge the PR I don't think I can publish an update to Rubygems.

[wldcordeiro]

@lorefnon register for rubygems and email me or ping me here and I can update you as an owner there too

[lorefnon]

@wldcordeiro Thanks for merging the PR.

My rubygems account is registered using the same email as the one on the github public profile: lorefnon@gmail.com

[wldcordeiro]

@lorefnon ownership was added. You should be able to publish new versions. Ping me if you have any questions. 👍