fix reshape on non-vector arrays

[johnnychen94]

A bug introduced by #88 😮

julia> using OffsetArrays

julia> A = rand(4, 4);

julia> reshape(A, :) |> axes
(Base.OneTo(16),)

julia> reshape(OffsetArray(A, 0, 0), :) |>  axes
(1:4, 1:4)

No new ambiguity here.

[codecov]

Codecov Report

Merging #151 into master will increase coverage by 0.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   93.60%   94.02%   +0.42%     
==========================================
  Files           2        2              
  Lines         250      251       +1     
==========================================
+ Hits          234      236       +2     
+ Misses         16       15       -1     

Impacted Files	Coverage Δ
src/OffsetArrays.jl	94.68% <100.00%> (+0.51%)	⬆️

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