* Fix for 3 Issues Involving ReadXlsx and NamedRange

Issues PHPOffice#1686 and PHPOffice#1723, which provide sample spreadsheets, are probably
solved by this ticket. Issue PHPOffice#1730 is also probably solved, but I have
no way to verify.

There are two problems with how PhpSpreadsheet is handling things now.
Although the first problem is much less severe, and isn't really a factor
in the issues named above, it is helpful to get it out of the way first.
If you define a named range in Excel, and then delete the sheet where
the range exists, Excel saves the range as #REF!. If there is a cell which
references the range, it will similarly have the value #REF! when you open
the Excel file.
Currently, PhpSpreadsheet discards the #REF! definition, so a cell which
references the range will appear as #NAME? rather than #REF!.
This PR changes the behavior so that PhpSpreadsheet retains the #REF!
definition, and cells which reference it will appear as #REF!.

The second problem is the more severe, and is, I believe, responsible
for the 3 issues identified above.
If you define a named range and the sheet on which the range is defined
does not exist at the time, Excel will save the range as something like:

'[1]Unknown Sheet'!$A$1

If a cell references such a range, Excel will again display #REF!.
PhpSpreadsheet currently throws an Exception when it encounters
such a definition while reading the file. This PR changes
the behavior so that PhpSpreadsheet saves the definition as #REF!,
and cells which reference it will behave similarly.

For the record, I will note that Excel does not magically recalculate when a
missing sheet is subsequently added, despite the fact that the reference
might now become resolvable. PhpSpreadsheet behaves likewise.

* Remove Dead Code in Test

Identified it after push but before merge.