Correction to t/overload_cross_class.t. Only relevant if a pre-4.19 version of Math::MPFR is loaded. Fixes https://www.cpantesters.org/cpan/report/a75afb86-6d32-1014-b14f-669fb03d33c9
When overloaded +=, -=, *=, /=, %=, and **= operations return a Math::MPFR object, the bit-precision of that returned Math::MPFR object is the same as the bit-precision of the Math::MPFR object that was provided as an argument. (Previously, these returned objects were given current Math::MPFR default precision.)