make check_fortran_type return false for inhomogeneous arrays

[sostock]

Right now, check_fortran_type(x::Array) checks the first element of x instead of its eltype. Therefore, it will return true for, e.g., Integer[Int32(1), BigInt(5)], since Int32 is a bitstype, but serializing the array will fail. This PR makes check_fortran_type return false in these cases.

[coveralls]

Coverage remained the same at 95.192% when pulling b6e9048 on sostock:sostock/inhom into f15021b on traktofon:master.

[codecov-io]

Codecov Report

Merging #4 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   92.91%   92.93%   +0.02%     
==========================================
  Files           9        9              
  Lines         268      269       +1     
==========================================
+ Hits          249      250       +1     
  Misses         19       19

Impacted Files	Coverage Δ
src/write.jl	94.73% <100%> (+0.14%)	⬆️

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[traktofon]

Thanks, that makes sense.