remove_alternation fails to detect non-weak automata

Discovered while discussing with @tmichaud.

The output is incorrect because the input is not weak and remove_alternation() is currently only expected to work on weak automata. An exception should be thrown, but somehow the non-weak detection code is flawed.

HOA: v1
States: 4
AP: 2 "a" "b"
Start: 0&1
Acceptance: 1 Inf(0)
--BODY--
State: 0 {0}
[0] 2
State: 2 /*{0}*/
[0] 0
State: 1 {0}
[1] 3 
State: 3 /*{0}*/
[1] 1
--END--