ext/bcmath: Fixed the sign to be PLUS if the result is 0

[SakiTakamachi]

This does not affect existing functions at all, but when comparing on BcMath\Number objects, mixing 0 and -0 will result in unintended behavior, so modify it so that the sign is always PLUS for 0 .

[cmb69]

Maybe add a test case for this?

[SakiTakamachi]

@cmb69
There is no effect on existing functions (because the calculation results are converted to strings each time instead of being retained as a structure), so cannot write tests on the current master.

edit:
I could include this in the RFC implementation PR, but since it's already over 5000 lines, I thought it might be better to separate it other PR.