Fix S1871 FN: Support single line conditional block

[fmallet]

RSPEC-3923 and RSPEC-1871 have been updated in sync to support two new exceptions (see the rule descriptions). The implementation of RSPEC-1871 must be updated accordingly.

Relates to #1079

[Evangelink]

Exceptions for S3923 were already handled in the implementation of #1079. I'll update S1871 from this ticket.