chore: relax HTTP header name validation to match RFC 7230

Updated the regex in `src/helpers.ts` to allow all valid characters in an HTTP token (RFC 7230, section 3.2.6), including symbols like `_`, `.`, `!`, and `*`. Previously, the validation was overly restrictive, only allowing alphanumeric characters and hyphens.

Also updated the corresponding unit test in `__tests__/helpers.test.ts` to reflect the change.

__tests__/helpers.test.ts

@@ -214,13 +214,11 @@ valid123: value5`
 
       expect(result).toEqual({
         'valid-header': 'value1',
+        invalid_underscore: 'value3',
         valid123: 'value5',
       })
 
       expect(core.warning).toHaveBeenCalledWith(expect.stringContaining('Skipping invalid header name: invalid header'))
-      expect(core.warning).toHaveBeenCalledWith(
-        expect.stringContaining('Skipping invalid header name: invalid_underscore'),
-      )
       expect(core.warning).toHaveBeenCalledWith(expect.stringContaining('Skipping invalid header name: invalid@header'))
     })