This PR is intended to fix #623.

Cause of the bug

Currently, styles of cells included in a merged cell are merged and always the same after reading from file, because Worksheet#set model() calls Cell#merge(), which overrides the cell's style with the master (left-top) cell.

However, in the original xml, each cell in a merged cell may have different styles.
For example, this file, which I created with Excel, has sheet1.xml like this:

  <sheetData>
    <row r="2" spans="2:3">
      <c r="B2" s="1"/>
      <c r="C2" s="2"/>
    </row>
    <row r="3" spans="2:3">
      <c r="B3" s="3"/>
      <c r="C3" s="4"/>
    </row>
  </sheetData>
  <mergeCells count="1">
    <mergeCell ref="B2:C3"/>
  </mergeCells>

Thus, merging the styles drops intended cell styles, which causes #623.

Solution

Worksheet#set model() is changed such that it parses merged cells without overriding the styles of included cells.

Note

I keep the behavior of Worksheet#mergeCells() unchanged, since this is an API that can be used programmatically. https://github.com/exceljs/exceljs#merged-cells
Another related issue #635 seems to correspond to this programmatic use case, but I think it can be tackled separately.