fix merging of initial states in state-based automata

Fixes #522 (closed) reported by Raven Beutner.

• spot/parseaut/parseaut.yy: Make sure all edges leaving the initial state have the same color.
• THANKS: Add Raven.
• NEWS: Mention the bug.
• tests/core/522.test: New file.
• tests/Makefile.am: Add it.