Tests added thanks to https://github.com/W1M0R

Export-Excel.Tests.ps1

@@ -0,0 +1,64 @@
+# Contributed by https://github.com/W1M0R
+
+Import-Module ImportExcel -Force 
+
+function New-TestWorkbook {
+    $testWorkbook = "$($PSScriptRoot)\test.xlsx"
+    
+    Remove-Item $testWorkbook -ErrorAction Ignore
+    $testWorkbook
+}
+
+function Remove-TestWorkbook {
+    New-TestWorkbook | Out-Null
+}
+
+function New-TestDataCsv {
+@"
+ID,Product,Quantity,Price,Total
+12001,Nails,37,3.99,147.63
+12002,Hammer,5,12.10,60.5
+12003,Saw,12,15.37,184.44
+01200,Drill,20,8,160  
+00120,Crowbar,7,23.48,164.36
+true,Bla,7,82,12
+false,Bla,7,82,12
+2009-05-01 14:57:32.8,Yay,1,3,2
+"@ | ConvertFrom-Csv 
+}
+
+Describe "Export-Excel" {
+
+    $csvData  = New-TestDataCsv
+    $workbook = New-TestWorkbook
+
+    Context "Importing CSV data from a here string" {
+        It "All properties are type [string]" {
+            $csvData | % {
+                $_.PSObject.Properties | % {
+                    $_.Value -is [string] | Should Be $true
+                }
+            }
+        }
+        It "Leading zeroes are preserved" {
+            $csvData[4] | Select-Object -ExpandProperty ID | Should Be "00120"
+        }
+    }
+
+    Context "Piping CSV data to Export-Excel" {
+
+        $xlPkg = $csvData | Export-Excel $workbook -PassThru
+        $ws = $xlPkg.Workbook.WorkSheets[1]
+
+        It "Exports numeric strings as numbers" {
+            $csvData[2] | Select-Object -ExpandProperty ID | Should Be "12003"
+            $ws.Cells["A4"].Value -is [double] | Should Be $true
+            $ws.Cells["A4"].Value | Should Be 12003
+        }
+
+        $xlPkg.Save()
+        $xlPkg.Dispose()
+    }
+
+    Remove-TestWorkbook
+}