Fix detect_compressed()

[alyst]

The PR fixes a typo in BZIP2 magic bytes that is responsible for JuliaData/RData.jl#83 and adds unit tests for all supported compression formats.

I've also refactored the compression detection: now there is detect_compressor() function that returns the compression format code or nothing if no supported format detected.
I would be using it from RData.jl instead of the compression detection code that is currently there to avoid code duplication.
Also, it could help in future when implementing the format detection of decompressed streams.
For example, the current version of FileIO checks for RData-specific magic bytes if the file is not compressed, but for compressed RData files it just checks the compression format but skips magic bytes check of the uncompressed stream (see also #225).

Once merged, it would be nice to tag a new version of FileIO, because the fix of JuliaData/RData.jl#83 depends on it.

[codecov]

Codecov Report

Merging #318 (26c3a19) into master (3e72cd4) will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
+ Coverage   89.88%   90.22%   +0.33%     
==========================================
  Files          10       10              
  Lines         623      624       +1     
==========================================
+ Hits          560      563       +3     
+ Misses         63       61       -2     

Impacted Files	Coverage Δ
src/registry.jl	90.54% <100.00%> (+0.74%)	⬆️
src/precompile.jl	100.00% <0.00%> (+6.25%)	⬆️

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[alyst]

@timholy could you please take a look?

[johnnychen94]

Curious to ask, will it be better to use :GZIP, :BZIP2, :XZ, :LZ4 and perhaps :plain (or :uncompressed) here?

[johnnychen94]

Once merged, it would be nice to tag a new version of FileIO, because the fix of JuliaData/RData.jl#83 depends on it.

FileIO v1.6.5 is out.

[alyst]

Curious to ask, will it be better to use :GZIP, :BZIP2, :XZ, :LZ4 and perhaps :plain (or :uncompressed) here?

Thanks for merging!
I think returning :uncompressed would be misleading as we only check a limited subset of compressors.
I don't have a particular preference to symbols or strings, I've just kept the current choice.