fix #1970 - make MultiPartFormDataContent open and exposed boundary

[jschneidereit]

Fixes #1970

Subsystem
Client

Motivation
Before this change MultiPartFormDataContent was not compatible with servers that accepted multipart/mixed content. By exposing boundary clients of ktor client can implement their own classes to override the content type boundary behavior like so:

class MultiPartMixedDataContent(
        parts: List<PartData>
) : MultiPartFormDataContent(parts) {
    override val contentType: ContentType = ContentType.MultiPart.Mixed.withParameter("boundary", boundary)
}

Solution
I made MultiPartFormDataContent an open class and exposed boundary to subclasses to allow for the above code snippet

[jschneidereit]

Hey there @e5l, any updates on this?