expand_derivatives returns float expression

[karlwessel]

The following MWE demonstrates a case where expand_derivatives returns a floating point constant instead of a rational:

julia> using Symbolics

julia> @variables a x
2-element Vector{Num}:
 a
 x

julia> ex = exp(a*x)/(2a)
exp(a*x) / (2a)

julia> D = Differential(x)
Differential(x)

julia> expand_derivatives(D(D(ex)))
0.5a*exp(a*x)

Note that expanding the derivative of the manual input of the result of the inner derivative correctly uses an rational:

julia> expand_derivatives(D(ex))
exp(a*x) / 2

julia> expand_derivatives(D(exp(a*x)/2))
(1//2)*a*exp(a*x)

This is likely because the manual input gets automatically changed to $\frac 12 \exp(ax)$ but the result of the expanded inner derivative is $\exp(ax)/2$.

Tested with Symbolics v6.22.1 and v6.10.0.