Fix unions for upgrades on mutable structs

[IanButterworth]

Fixes #628

It's a bit of a guess, but tests pass for me locally, including the test added here, which reproduces the issue on master for mutable structs.

On master

Error encountered while load File{DataFormat{:JLD2}, String}("a.jld2").

Fatal error:
mutable: Error During Test at /Users/ian/Documents/GitHub/JLD2.jl/test/loadsave.jl:750
  Got exception outside of a @test
  TypeError: in Union, expected Type, got a value of type JLD2.Upgrade
  Stacktrace:
    [1] jlconvert(::JLD2.MappedRepr{Union, JLD2.OnDiskRepresentation{(0, 16, 32), Tuple{String, Vector{Any}, Vector{Any}}, Tuple{JLD2.Vlen{String}, JLD2.Vlen{JLD2.RelOffset}, JLD2.Vlen{JLD2.RelOffset}}, 48}}, f::JLD2.JLDFile{JLD2.MmapIO}, ptr::Ptr{Nothing}, header_offset::JLD2.RelOffset)
      @ JLD2 ~/Documents/GitHub/JLD2.jl/src/data/writing_datatypes.jl:549
    [2] read_scalar(f::JLD2.JLDFile{JLD2.MmapIO}, rr::Any, header_offset::JLD2.RelOffset)
      @ JLD2 ~/Documents/GitHub/JLD2.jl/src/io/dataio.jl:107
...
Test Summary:                                     | Pass  Error  Total  Time
Issue #628 Upgrading structs that contains unions |    3      1      4  1.8s
  immutable                                       |    3             3  0.4s
  mutable                                         |           1      1  1.0s
ERROR: LoadError: Some tests did not pass: 3 passed, 0 failed, 1 errored, 0 broken.
in expression starting at /Users/ian/Documents/GitHub/JLD2.jl/test/loadsave.jl:740
in expression starting at /Users/ian/Documents/GitHub/JLD2.jl/test/runtests.jl:25
ERROR: Package JLD2 errored during testing

cc. @Octogonapus

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.81%. Comparing base (b8b0f9a) to head (fb669c3).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #629      +/-   ##
==========================================
+ Coverage   84.77%   84.81%   +0.04%     
==========================================
  Files          37       37              
  Lines        4242     4241       -1     
==========================================
+ Hits         3596     3597       +1     
+ Misses        646      644       -2     

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[JonasIsensee]

Thanks!
This is probably what I would have done as well.

This change make the checking for Upgrade in line 357 unnecessary.
That should probably be removed together with this change.