Bug fix for bad "dimension" spec.

[berntb]

Some programs (Libre Office) create XLSX files with a bad dimension spec. They only save max col/row.

This fix to Spreadsheet/ParseXLSX.pm should illustrate (and solve) the problem:

sub _dimensions {
     my $self = shift;
     my ($dim) = @_;
 
+    $dim  = "A1:$dim"
+        if $dim !~ /:/;