Conversion tests + bugfix for Unit{Upper/Lower}Triangular

[kshyatt]

Converting from Diagonal to UnitUpperTriangular or UnitLowerTriangular wasn't tested and had bugs. In particular, the Unit...Triangular types have unit diagonals, which wasn't checked for at all.

[tkelman]

Have we mentioned enough times how great these are? You're even chipping away bit by bit at JuliaLang/LinearAlgebra.jl#136 which is really good to see.

I think the unit triangular types don't exist in release-0.3 so the bugfix part of this is not a backport candidate, but correct me if I'm wrong.

[andreasnoack]

Yes, it is really a great job. The unit triangular matrices also exist in 0.3, but there the unit diagonal is marked by a parameter. Hence, it will probably be a bit cumbersome to backport, but we should try fix the bug anyway.

[tkelman]

okay it'll probably have to be fixed by hand then, but i'll add the label here until then so it doesn't get forgotten

[andreasnoack]

The conversion to unit triangular is not defined in 0.3, so there is not a bug there. Unless someone asks for it, I don't think we should add the conversion so I'm removing the backport label.