Fixed error in "The Quotient Rule"

Differentiation of a fraction is wrong if the numerator or denominator independent of the variable differentiated with respect to.
It looks like somebody forgot to divide by (denominator)^2 for the special cases where one of the derivatives is 0

Author: ivarne

src/differentiate.jl

@@ -105,9 +105,9 @@ function differentiate(::SymbolParameter{:/}, args, wrt)
     if xp == 0 && yp == 0
         return 0
     elseif xp == 0
-        return :( -$yp * $x )
+        return :( -$yp * $x / $y^2 )
     elseif yp == 0
-        return :( $xp * $y )
+        return :( $xp / $y )
     else
         return :( ($xp * $y - $x * $yp) / $y^2 )
     end