fix posdef check for matrices + add tests

[dpo]

fixes #102

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #105   +/-   ##
======================================
  Coverage    93.4%   93.4%           
======================================
  Files           5       5           
  Lines         500     500           
======================================
  Hits          467     467           
  Misses         33      33

Impacted Files	Coverage Δ
src/LinearOperators.jl	92.8% <100%> (ø)	⬆️

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[abelsiqueira]

Thanks.