simplify push_back logic

[unixnme]

Hello boost devs/maintainers,

I find the current push_back logic a bit strange with the two-steps

• call resize() method but using the default value = false
• then call set() method to explicitly set the bit

I feel it would make sense to simplify this with a single step and reduce overhead

• call resize() method with the given bit

I'm not sure if this is intended, but if not, here is my pull request to address the issue.

Thanks.