`isapprox` for heterogeneous arrays

`isapprox` behaves incorrectly for heterogeneous arrays of quantities:
```julia
julia> a = Quantity[1u"s", 1u"minute"]
2-element Array{Quantity,1}:
1 s
1 minute

julia> isapprox(a,a)
false
```
This is due to the following implementation:
https://github.com/PainterQubits/Unitful.jl/blob/f6866eaffbb3a9150921cdcdacc967e8ff4a5c03/src/quantities.jl#L247-L248

Reproduce the heterogeneous-array example in Julia, then inspect the implementation linked at src/quantities.jl lines 247-248. Confirm the fix by checking that isapprox(a, a) returns true for arrays containing quantities with different units.

Written by the indexing model from the issue text.