[Mime] Fix boundary header

[guillbdx]

Q	A
Branch?	4.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #35443 (fixes the second problem described in this ticket)
License	MIT

The boundary value of Content-Type header was enclosed in quotes, cause of the "=" symbol.

[fabpot]

What does it fix?

[guillbdx]

Description updated.

[fabpot]

As far as I know, the current chars used in the boundary are correct, so I would prefer that we fix the root cause instead.

[guillbdx]

Yes, after checking, the chars are correct, and the quotes should not cause any problem at all, that's even a recommandation:

The grammar for parameters on the Content-type field is such that it is often necessary to enclose the boundary parameter values in quotes on the Content-type line. This is not always necessary, but never hurts.
https://tools.ietf.org/html/rfc2046#section-5.1.1

So I assume the root cause is API side in the ticket #35443 and there's not much we can do.

[n3o77]

I think the actual problem here is that the boundary is only put in quotes in the header but not the parts in the message (as described here: #35443 (comment) )

[guillbdx]

@n3o77 It's normal, quotes are only for the parameter value in the header: https://tools.ietf.org/html/rfc2046#section-5.1.1

Anyway, this PR should fix this interoperability issue.

[fabpot]

Thank you @guillbdx.