Fix incorrect isweak default

Author: jpsamaroo

src/thunk.jl

@@ -247,7 +247,7 @@ wrap_weak(t::WeakThunk) = t
 wrap_weak(t) = t
 isweak(t::WeakThunk) = true
 isweak(t::Thunk) = false
-isweak(t) = true
+isweak(t) = false
 Base.show(io::IO, t::WeakThunk) = (print(io, "~"); Base.show(io, t.x.value))
 Base.convert(::Type{WeakThunk}, t::Thunk) = WeakThunk(t)
 chunktype(t::WeakThunk) = chunktype(unwrap_weak_checked(t))