Broadcast with Bool and Array causes error

The following example fails:
```julia
using Tracker
f(x, ::Bool) = x;
x = rand(3);
F = x -> sum(f.(x, false));
Tracker.gradient(F, x)
```
The error is:
```julia
ERROR: MethodError: no method matching f(::ForwardDiff.Dual{Nothing,Float64,1}, ::ForwardDiff.Dual{Nothing,Bool,1})
Closest candidates are:
f(::Any, ::Bool) at REPL[3]:1
Stacktrace:
[1] partial(::typeof(f), ::Float64, ::Int64, ::Float64, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\array.jl:480
[2] _broadcast_getindex at .\broadcast.jl:578 [inlined]
[3] getindex at .\broadcast.jl:511 [inlined]
[4] copy at .\broadcast.jl:787 [inlined]
[5] materialize(::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1},Nothing,typeof(Tracker.partial),Tuple{Base.RefValue{typeof(f)},Array{Float64,1},Int64,Array{Float64,1},Bool}}) at .\broadcast.jl:753
[6] broadcast(::typeof(Tracker.partial), ::Base.RefValue{typeof(f)}, ::Array{Float64,1}, ::Int64, ::Vararg{Any,N} where N) at .\broadcast.jl:707
[7] ∇broadcast(::typeof(Tracker.partial), ::Base.RefValue{typeof(f)}, ::Array{Float64,1}, ::Int64, ::TrackedArray{…,Array{Float64,1}}, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\array.jl:484
[8] copy(::Base.Broadcast.Broadcasted{Tracker.TrackedStyle,Tuple{Base.OneTo{Int64}},typeof(Tracker.partial),Tuple{Base.RefValue{typeof(f)},Array{Float64,1},Int64,TrackedArray{…,Array{Float64,1}},Bool}}) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\array.jl:515
[9] materialize at .\broadcast.jl:753 [inlined]
[10] #547 at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\array.jl:488 [inlined]
[11] ntuple at .\tuple.jl:160 [inlined]
[12] (::getfield(Tracker, Symbol("#back#548")){2,typeof(f),Tuple{TrackedArray{…,Array{Float64,1}},Bool}})(::Array{Float64,1}) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\array.jl:488
[13] back_(::Tracker.Call{getfield(Tracker, Symbol("#back#548")){2,typeof(f),Tuple{TrackedArray{…,Array{Float64,1}},Bool}},Tuple{Tracker.Tracked{Array{Float64,1}},Nothing}}, ::Array{Float64,1}, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:35
[14] back(::Tracker.Tracked{Array{Float64,1}}, ::Array{Float64,1}, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:58
[15] foreach at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:38 [inlined]
[16] back_(::Tracker.Call{getfield(Tracker, Symbol("##482#483")){TrackedArray{…,Array{Float64,1}}},Tuple{Tracker.Tracked{Array{Float64,1}}}}, ::Float64, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:38
[17] back(::Tracker.Tracked{Float64}, ::Int64, ::Bool) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:58
[18] #back!#15 at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:77 [inlined]
[19] #back! at .\none:0 [inlined]
[20] #back!#32 at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\real.jl:16 [inlined]
[21] back!(::Tracker.TrackedReal{Float64}) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\lib\real.jl:14
[22] gradient_(::Function, ::Array{Float64,1}) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:4
[23] #gradient#24 at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:164 [inlined]
[24] gradient(::Function, ::Array{Float64,1}) at C:\Users\user\.julia\packages\Tracker\RRYy6\src\back.jl:164
[25] top-level scope at none:0
```