Weird attribute caching bug brings values back from the dead

[david-macmahon]

Using HDF5.jl v0.17.2, attributes seem to have some weird caching problem that brings old values back from the dead. To recreate:

1. Create/set an attribute to one value
2. Delete the attribute
3. Create/set the same attribute to a different value
4. Reading the attribute value returns the originally written value, not the new value
5. Closing and re-opening the file and re-reading the attribute gets the new value

julia> h5 = h5open("test.h5", "w")
🗂️ HDF5.File: (read-write) test.h5

julia> h5["data"] = rand(4,3);

julia> h5["data"]["attr"] = "foo"
"foo"

julia> h5
🗂️ HDF5.File: (read-write) test.h5
└─ 🔢 data
   └─ 🏷️ attr

julia> h5["data"]["attr"][]
"foo"

julia> delete_attribute(h5["data"], "attr")

julia> h5
🗂️ HDF5.File: (read-write) test.h5
└─ 🔢 data

julia> h5["data"]["attr"][]
ERROR: KeyError: key "attr" not found
Stacktrace:
 [1] getindex(dset::HDF5.Dataset, name::String)
   @ HDF5 ~/.julia/packages/HDF5/Z859u/src/attributes.jl:403
 [2] top-level scope
   @ REPL[119]:1

julia> h5["data"]["attr"] = "bar"
"bar"

julia> h5["data"]["attr"][] # Should return "bar"!!!
"foo"

julia> close(h5)

julia> h5 = h5open("test.h5")
🗂️ HDF5.File: (read-only) test.h5
└─ 🔢 data
   └─ 🏷️ attr

julia> h5["data"]["attr"][]
"bar"