fix Issue 13252 - ParameterDefaultValueTuple affects other instantiations

Expression::equals should also compare operand types.
Remove toHeadMutable call from NullExp::equals and ArrayLiteralExp::equals

I'm not sure we should also remove toHeadMutable call from IntergerExp, RealExp, ComplexExp, and VarExp::equals. To avoid accidental existing code breaking, I'd leave them in 2.066 beta/rc phase.
Merge pull request #3846 from 9rnsr/fix13252

[REG2.061] Issue 13252 - ParameterDefaultValueTuple affects other instantiations