Marshaler / Reflector tries to convert time.Time struct into fix format

Created by: daino3

Marshaling MarketDataFullRefresh with NoMDEntries with the following:

fullRefresh, err := book.ToFixMarketDataFullRefreshBidsAndOffers(mdReqID) // err is nil
qfMsg = fullRefresh.Marshal()

Panics and supplies the following backtrace:

•! Panic [0.000 seconds]
      Test Panicked
      strconv.ParseInt: parsing "": invalid syntax
      /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:426

      Full Stack Trace
        /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:426 +0x4e9
      github.com/quickfixgo/quickfix.parseStructTag(0x0, 0x0, 0x4ee280, 0x0, 0x0)
        /Users/daino/Programs/go/src/github.com/quickfixgo/quickfix/reflection.go:12 +0x101
      github.com/quickfixgo/quickfix.buildGroupTemplate.func1(0xb04080, 0x4de680)

Perpetrator: https://github.com/quickfixgo/quickfix/blob/master/fix50sp1/mdfullgrp/MDFullGrp.go#L57