fix: make extra log arguments as attributes

[violin0622]

It seems gorm slog changed log format, making those attributes as part of the message.
I dont' think this is good idea.
For example, This is what slog-gorm v1.3.0 does(when formatted by devslog handler):

With a previous version of slog-gorm, it looks like:

Definitely the second one is better.

So I make this PR to recover previous behaviour.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d05e02a) to head (027a8b0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          152       154    +2     
=========================================
+ Hits           152       154    +2     

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[orandin]

Thank you @violin0622 for your contribution 👍

Your contribution only works for the Trace method. The methods Info, Warn & Error must use fmt.Sprintf to respect the Writer interface (#15). It seems that the PR #21 manage these different cases.

[orandin]

Fixed by #21