Fix dot head

[t-bltg]

While working on this example (to bypass JuliaLang/julia#18466), I encountered an issue using Base.RefValue:

using ComputedFieldTypes
import Base: RefValue

@computed struct Foo{T <: AbstractFloat, N}
  a::Array{T, N}
  b::Array{T, N + 1}
  Foo(a::Array{T, N}) where {T, N} = new{T, N}(a, zeros((2, size(a)...)))
end

struct Bar{T <: AbstractFloat, N}
  # r::RefValue{Foo{T, N}}  # obviously fails, but should it ? (fixed by ComputedFieldTypes.jl/pull/20)
  r::RefValue{Foo{T, N, 3}}  # works, but what's the point of using @computed then ? 
  Bar(r::Foo{T, N}) where {T, N} = new{T, N}(Ref(r))
end

@computed struct Baz{T <: AbstractFloat, N}
  r::Base.RefValue{Foo{T, N, N + 1}}  # <==== fails without this PR
  Baz(r::Foo{T, N}) where {T, N} = new{T, N}(Ref(r))
end

main() = begin
  foo = Foo([0. 1.])
  Bar(foo)
  Baz(foo)
  return
end

main()