Fix CopyTo same filepath truncate the file

[alessio-perugini]

Issue

When trying to copy the same file from src to dst the CopyTo functions truncate
the dst file.

Example:
src=dst=/tmp/a

/tmp/a -> content: "hello"
src.CopyTo(dst)
/tmp/a -> content: ""

New behaviour

Copying an existing file to the same directory throws an error.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.43% 🎉

Comparison is base (7d88805) 71.95% compared to head (38b01d1) 72.39%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   71.95%   72.39%   +0.43%     
==========================================
  Files           6        6              
  Lines         353      355       +2     
==========================================
+ Hits          254      257       +3     
+ Misses         67       66       -1     
  Partials       32       32              

Flag	Coverage Δ
unit	72.39% <100.00%> (+0.43%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
paths.go	64.08% <100.00%> (+0.95%)	⬆️

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