Fix divmod and modulo by infinity to match Float#divmod and Float#modulo

[tompng]

Fix this bug

(-2.0).divmod(Float::INFINITY)
#=> [-1, Infinity]

BigDecimal(-2).divmod(BigDecimal::INFINITY)
#=> [0.0, -0.2e1] 

2.0.divmod(-Float::INFINITY)
#=> [-1, -Infinity]

BigDecimal(2).divmod(-BigDecimal::INFINITY)
#=> [0.0, 0.2e1]