Fix #72964: White space not unfolded for CC/Bcc headers

`\r\n` does only terminate a header, if not followed by `\t` or ` `.
We have to cater to that when determining the end position of

Fix #72964: White space not unfolded for CC/Bcc headers

`\r\n` does only terminate a header, if not followed by `\t` or ` `.
We have to cater to that when determining the end position of the
respective headers.

Closes GH-6420.

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