Changed bitset operator overloads from bitwise operators to logical assignments.

[psmskelton]

Changed the Bitset &=, |=, and ^= operator overloads to logical assignments instead of bitwise operators to fix scan warnings.

Since C++11, these are guaranteed to be equivalent operations, except LHS is only evaluated once instead of twice.

[psmskelton]

Interestingly, I cannot reproduce the scan problems with lua-protobuf on my end.