Update natural-sorting.js

[usmanpakistan]

I was having an issue with sorting negative numbers. Because the regex used below strip negative numbers and convert them to a positive number. To solve this issue I have replaced line 58 to handle negative numbers. Fixes https://github.com/wenzhixin/bootstrap-table-examples/blob/master/utils/natural-sorting/src/natural-sorting.js

[smillove]

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[smillove]

这是一封自动回复邮件。已经收到您的来信，我会尽快回复。