marshaling complex messages error

Created by: babaev

Tests don`t cover the case when marshaling messages with repeating groups which have embedded structs and tags with multiple values.

Example:

    m := &marketdatarequest.Message{}
    m.SetMDReqID(myPrefix)
    m.SetSubscriptionRequestType(Type)
    m.SetMarketDepth(1)
    m.SetMDUpdateType(0)
    m.SetNoMDEntryTypes([]marketdatarequest.NoMDEntryTypes{
        marketdatarequest.NoMDEntryTypes{enum.MDEntryType_BID},
        marketdatarequest.NoMDEntryTypes{enum.MDEntryType_OFFER},
    })
    nRelatedSym := marketdatarequest.NoRelatedSym{}
    nRelatedSym.Symbol = &sPair
    m.SetNoRelatedSym([]marketdatarequest.NoRelatedSym{
        nRelatedSym,
    })

    message := m.Marshal()

This code will cause panic