Unset the optional bit for a feature when setting the required bit

There is no reason to set both, and this currently makes testing
the new BOLT invoice tests slightly harder, so we just unset it.

Author: TheBlueMatt

lightning/src/ln/features.rs

@@ -316,6 +316,7 @@ mod sealed {
 					}
 
 					flags[Self::BYTE_OFFSET] |= Self::REQUIRED_MASK;
+					flags[Self::BYTE_OFFSET] &= !Self::OPTIONAL_MASK;
 				}
 
 				/// Sets the feature's optional (odd) bit in the given flags.