Enable branch coverage measurement

Simplecov doc:
https://github.com/simplecov-ruby/simplecov#branch-coverage-ruby--25

Ruby feature:
https://bugs.ruby-lang.org/issues/13901

Author: mvidner

spec/spec_helper.rb

@@ -15,6 +15,8 @@
   SimpleCov.add_filter "_spec.rb"
   # do not cover the activesupport helpers
   SimpleCov.add_filter "/core_ext/"
+  # measure all if/else branches on a line
+  SimpleCov.enable_coverage :branch
 
   SimpleCov.start