Allow explicit non string types through without being cast to empty strings

[incentify-dev]

This is a proposed fix for open issue #103

This config option is OFF by default (including old configs not updated). Test have been added.

[codecov-io]

Codecov Report

Merging #148 (bc12b36) into master (8e0b3d8) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #148      +/-   ##
============================================
+ Coverage     88.18%   88.28%   +0.10%     
- Complexity       46       48       +2     
============================================
  Files             4        4              
  Lines           110      111       +1     
============================================
+ Hits             97       98       +1     
  Misses           13       13              

Impacted Files	Coverage Δ	Complexity Δ
src/Purifier.php	89.36% <100.00%> (+0.23%)	41.00 <0.00> (+2.00)
src/PurifierServiceProvider.php	85.71% <0.00%> (-0.96%)	6.00% <0.00%> (ø%)

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[incentify-dev]

I wanted to add a little more information about this. With the default config out of this box for this library, see the below code block showing the input/output when explicit types are passed in (Imagine a use case like sanitizing the entire Laravel Input array):

$input = [
      'good'=>'<span id="some-id">This is my H1 title',
      'bad'=>'<script>alert(\'XSS\');</script>',
      'empty'=>null,
      'bool'=>false,
      'bool2'=>true,
      'float'=>4.321,
  ];
  $output = [
      'good'=>'<p><span>This is my H1 title</span></p>',
      'bad'=>'',
      'empty'=>'',
      'bool'=>'',
      'bool2'=>'<p>1</p>',
      'float'=>'<p>4.321</p>'
  ];

For the APIs I'm using this project with, this loss of explicit type is destructive, and it seems reasonable that there should be a configuration option to bypass purification for non string values. I'm open to other approaches, and perhaps being more restrictive, only passing through types which can pass explicit built in checks (eg: is_int, is_float, is_bool, is_null, etc)