fix #1970 - update MultiPartFormDataContent to allow contentType override using optional builder

[jschneidereit]

Fixes #1970

Subsystem
Client

Motivation
Before this change MultiPartFormDataContent was not compatible with servers that accepted multipart/mixed content.

Solution
Now a user can pass their own lambda for building content type like so:

val mixed = MultiPartFormDataContent(
    parts = listOf(),
    buildContentType = { b -> ContentType.MultiPart.Mixed.withParameter("boundary", b) }
)

[e5l]

Hey @jschneidereit, sorry for the delay. Could you check one thing before the merge?

[jschneidereit]

Hey @jschneidereit, sorry for the delay. Could you check one thing before the merge?

@e5l did I miss something?

[e5l]

Could you tell me why do we need boundary in the constructor?

[jschneidereit]

Hi @e5l I had originally made it override-able but @cy6erGn0m requested an optional constructor argument in the discussion in #1970

[jschneidereit]

The other option to only having contentType in the constructor and not boundary would be a lambda that accepts a string like contentTypeFn: (String) -> ContentType but I think that's a little overkill.