Adding test case for colums without 'rel' attribute

src/main/java/com/github/pjfanning/xlsx/impl/StreamingSheetReader.java

@@ -144,7 +144,8 @@ && isSpreadsheetTag(event.asStartElement().getName())) {
 
         if(ref != null) {
           String[] coord = splitCellRef(ref.getValue());
-          currentCell = new StreamingCell(currentRow, CellReference.convertColStringToIndex(coord[0]), Integer.parseInt(coord[1]) - 1, use1904Dates);
+          currentColNum = CellReference.convertColStringToIndex(coord[0]);
+          currentCell = new StreamingCell(currentRow, currentColNum, Integer.parseInt(coord[1]) - 1, use1904Dates);
         } else {
           currentCell = new StreamingCell(currentRow, currentColNum, currentRowNum, use1904Dates);
         }

src/test/java/com/github/pjfanning/xlsx/StreamingReaderTest.java

@@ -728,4 +728,26 @@ public void testFormulaWithDifferentTypes() throws Exception {
       assertThat(cell.getCachedFormulaResultType(), is(CellType.NUMERIC));
     }
   }
+
+  @Test
+  public void testShouldIncrementColumnNumberIfExplicitCellAddressMissing() throws Exception {
+    // On consecutive columns the <c> element might miss an "r" attribute, which indicate the cell position.
+    // This might be an optimization triggered by file size and specific to a particular excel version.
+    // The excel would read such a file without complaining.
+    try(
+            InputStream is = new FileInputStream(new File("src/test/resources/sparse-columns.xlsx"));
+            Workbook wb = StreamingReader.builder().open(is);
+    ) {
+      Sheet sheet = wb.getSheetAt(0);
+
+      Iterator<Row> rowIterator = sheet.rowIterator();
+      Row row = rowIterator.next();
+
+      assertThat(row.getCell(0).getStringCellValue(), is("sparse"));
+      assertThat(row.getCell(3).getStringCellValue(), is("columns"));
+      assertThat(row.getCell(4).getNumericCellValue(), is(0.0));
+      assertThat(row.getCell(5).getNumericCellValue(), is(1.0));
+
+    }
+  }
 }