support files without default namespace

[hhaensel]

I have an xlsx file that is generated by a website and can be read by both Excel and openpyxl.
However, I receive a "no node name" error when trying to parse the file with XLSX.
Unfortunately, I am not able to share the file at the moment.
But I succeeded in finding out the underlying root cause.

The file uses a non-default namespace, therefore the node names were prefixed with the namespace ("x:row" instead of "row"). Furthermore, there was no default namespace included in the worksheets styles, but only the namespace "x"=>"http://schemas.openxmlformats.org/spreadsheetml/2006/main"

This PR splits off the namespace for nodename checks and accepts a single namespace for assertion.
However, there is no checking for the correct namespace. But since there is only one namespace present, it is probably ok to handle the namespace this way.

[hhaensel]

For debugging I used the following lines. It could be a good starting point if in future we may want to match the namespace.

xf = XLSX.openxlsx(filename, enable_cache=false)

name_sheet1 = XLSX.sheetnames(xf)[1]
sheet = xf[name_sheet1]
itr = XLSX.eachrow(sheet)

ws = XLSX.get_worksheet(itr)
wb = XLSX.get_workbook(ws)

target_file = XLSX.get_relationship_target_by_id("xl", XLSX.get_workbook(ws), ws.relationship_id)
zip_io, reader = XLSX.open_internal_file_stream(XLSX.get_xlsxfile(ws), target_file)

# repeat execution of the following block for debugging 
begin
    r = iterate(reader)
    nodedepth(reader), nodename(reader)
end