In the section 3.2.6 of the HTTP RFC7230, it denotes the following about HTTP headers:

Most HTTP header field values are defined using common syntax
components (token, quoted-string, and comment) separated by
whitespace or specific delimiting characters. Delimiters are chosen
from the set of US-ASCII visual characters not allowed in a token
(DQUOTE and "(),/:;<=>?@[]{}").

token          = 1*tchar

tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
               / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
               / DIGIT / ALPHA
               ; any VCHAR, except delimiters

Note the tchar definition allows use of an exclamation mark, but the validator for normalizeName does not allow it.

This PR adds the exclamation mark to the list of allowed characters to header fields.

Verification

Execute the following in the browser console:

const headers = new Headers();
headers.append('test!', '123'); // Executes fine
headers.append('test:', '123'); // Throws a TypeError