Call user function only once in mean

Use an internal type to initiate promotion instead of using the type
returned by user-provided function applied to the first element.
This avoid calling the function twice, which can be confusing.

src/Statistics.jl

@@ -178,7 +178,10 @@ if !isdefined(Base, :mean)
     """
     mean(A::AbstractArray; dims=:) = _mean(identity, A, dims)
 
+    struct _InitType end
+
     _mean_promote(x::T, y::S) where {T,S} = convert(promote_type(T, S), y)
+    _mean_promote(x::_InitType, y::Any) = y/1
 
     # ::Dims is there to force specializing on Colon (as it is a Function)
     function _mean(f, A::AbstractArray, dims::Dims=:) where Dims
@@ -188,8 +191,7 @@ if !isdefined(Base, :mean)
         else
             n = mapreduce(i -> size(A, i), *, unique(dims); init=1)
         end
-        x1 = f(first(A)) / 1
-        result = sum(x -> _mean_promote(x1, f(x)), A, dims=dims)
+        result = sum(x -> _mean_promote(_InitType(), f(x)), A, dims=dims)
         if dims === (:)
             return result / n
         else