- The bug was inspired by N1 codec migration to the new version of MED.
As it was discovered, a med::choice with a compound header could not be
placed into a med::set or med::sequence as a mandatory member.
- It happened because of a process of postponed compound header tag filling.
A tag was filled out during a med::choice encode process, just before
encoding. That is, by the moment an outer container encoding, the choice
header tag remained untouched. Hereby, pre-encode check of all mandatory
fields filled caused an error reporting the emptyness of the mandatory
field.
- The fix moves the compound header tag filling from the encode stage to
 the earlier stage, namely while choice::ref() API is called.
- this change a protocol of working with a choice. Now a user can call
med::choice::ref() only once. To reuse a choice / refill some choice
member, med::choice::clear() must be called prior to it..
- The change also requires to distinguish a context within the ref()
 call happens. The decode stage, when the choice::ref() API is called
by the engine, is the case. During the choice::ref call the header tag
was already filled by the recent binary data reading and must not be
touched.
- To achieve it the new member method bool is_tag_set() was introduced.
 The new method must be presented in every class/struct where the
get_tag() method is defined.