Remove spaces from base64-encoded content

[Peter2121]

Some software creates mails with multi-line base64-encoded content with spaces inside (at the beginning of lines). Golang base64 decoder does not remove them, so such content is not decoded properly. It seems that this problem will be fixed in Golang 1.26. This PR fixes the problem, removing spaces from base64-encoded content.

[mnako]

Thank you for this contribution, @Peter2121 .

If you could add test cases following our existing patterns to show an example of base64-encoded content that needs spaces removed we will be happy to review this PR.