Refuse a second assignment to the same property in Fbe.if_absent and Fbe.just_one

[Thayorns]

The object that Fbe.if_absent and Fbe.just_one pass to their block kept the values in a Hash, so assigning the same property twice silently dropped the first value. A real fact would keep both, and both helpers already refuse an Array because they match by one value per property.

A second assignment to a property now raises Fbe::Error, naming the property, the new value and the one it already has, and nothing is inserted. The tests cover both the error and the empty factbase afterwards, for each helper.

To keep the complexity of both methods where it was, the empty string check next to the new line is written as v == '' instead of v.is_a?(String) && v.empty?. It means the same and has one branch less. The perceived complexity of if_absent is still 17 of 15, exactly as on master; this change does not touch that.

Closes #1244

[Thayorns]

@yegor256 take a look please, happy to clarify anything about the change.