Don't check magic bytes on save (fixes #267)

[timholy]

This allows query to return a format based only on the extension, even if the file already exists. The use-case is for save which just overwrites the file; in such cases, it doesn't really make sense to use any pre-existing magic bytes.

CC @dbohdan

[codecov]

Codecov Report

Merging #270 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
+ Coverage   76.31%   76.42%   +0.11%     
==========================================
  Files           8        8              
  Lines         418      420       +2     
==========================================
+ Hits          319      321       +2     
  Misses         99       99              

Impacted Files	Coverage Δ
src/loadsave.jl	87.01% <100.00%> (+0.17%)	⬆️
src/query.jl	87.01% <100.00%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a27ace...131461a. Read the comment docs.