Respect xml:space="preserve" (#43)

[TimG1964]

This PR fixes the issue reported in #43. I've added extra tests, too.

One caveat in testing: I had to change one test (only):

    @testset "next and prev" begin
        @test XML.prev(doc[1]) === data

I had to change the condition from === to only ==. With this single change, tests now all pass locally.

I've also checked that this PR works well with XLSX.jl downstream and all XLSX tests also pass locally.

With this PR, XLSX now properly reads a workbook like this:

which has leading and trailing whitespace in column A and, in column B, only has whitespace.
Reading this XLSX file now works correctly:

julia> f=XLSX.openxlsx(raw"C:\Users\tim\OneDrive\Documents\Julia\XLSX\sstTest.xlsx", mode="rw")
XLSXFile("sstTest.xlsx") containing 1 Worksheet
            sheetname size          range        
-------------------------------------------------
               Sheet1 5x2           A1:B5        

julia> f[1][:]
5×2 Matrix{Any}:
 "  hello"    "    "
 "  hello  "  "    "
 " hello\">"  "    "
 "hello\">"   "    "
 "  hello"    "    "

Previously, leading and trailing spaces would be trimmed, and cells containing only whitespace would become missing.

I'm afraid I can't match the simple elegance of your coding style, though!