This explicitly defines push_back and push_back methods for DataFrame
which copies row.names and class attributes from the original object.

This fixes #1094 where these operations converted the object to a list.

Tests for this behavior are also implemented.

Make use of the fact set__() already does a conversion, and just use
that. This has the benefit that it works with push_back(DataFrame), as
well as push_back(Vector), and also handles adding vectors of different
lengths properly.

With this commit, if after a push_back the row count of the DataFrame would
be invalid, a warning is generated. The type is then not coerced to a
DataFrame, but left as a list.

In future, this could instead `stop()`.

This triggers the warning when adding a zero-length object to a
DataFrame. It is not a valid DataFrame, so calls `warning()`, and doesn't
call `as.data.frame()`.