Added test for recently fixed sequence attribute consistency bug
Fix for sequences with only one element producing attr

If in a sequence only one element produces attribute, then make whole
sequence to produce that attribute,rather than wrapping it into
fusion::deque. Not doing so breaks alternative<> parser, when such
sequences are on left and right side of it, because it then generates
variant attribute over fusion:deque<> attributes, which is not
what it is supposed to do. Plus it breaks traits::move_to when both
source and destination are single element fusions sequences. Hence this
fix to put everything back to normal for this edge case.
Merge branch 'fix-one-element-seq' into parse-into-fusion-map
Support alternative over keys to be used as a key for fussion assoc

Adds support for parsers like:
auto key1 = lit("key1=") >> attr(key1());
auto key2 = lit("key2=") >> attr(key2());
auto value = int();
auto pair = key1|key2 >> value;

where attribute passed to pair.parse() is fusion associative sequence