Triangular matrix indexing bug

[bkalpert]

Index ranges seem to fail with triangular matrices:

julia> s=randn(6,6); s = s*s'
6x6 Array{Float64,2}:
 12.6695    4.37903    3.78534   -1.82609   9.07167   1.5337  
  4.37903   4.03807    2.3878    -3.87199   2.81139  -0.362388
  3.78534   2.3878     3.7409    -2.44415   2.95657   0.558897
 -1.82609  -3.87199   -2.44415    8.43938  -2.7138    1.60016
  9.07167   2.81139    2.95657   -2.7138    7.78165   0.22353
  1.5337   -0.362388   0.558897   1.60016   0.22353   2.67511

julia> u=chol(s)
6x6 UpperTriangular{Float64,Array{Float64,2}}:
 3.55943  1.23026  1.06347   -0.513029   2.54863    0.430883
 0.0      1.58887  0.679383  -2.0397    -0.20398   -0.561711
 0.0      0.0      1.46573   -0.349872   0.262503   0.329038
 0.0      0.0      0.0        1.97317   -0.87701    0.400681
 0.0      0.0      0.0        0.0        0.637546  -1.13589
 0.0      0.0      0.0        0.0        0.0        0.784136

julia> u[3:5,6]
ERROR: ArgumentError: Triangular matrix must have two dimensions
 in similar at linalg/triangular.jl:27
 in _unsafe_getindex at multidimensional.jl:193
 in getindex at abstractarray.jl:488

This was in version 0.4, but the same error occurs in today's 0.5.

[tkelman]

see #13731