Eg, In one cell the text is abcdef, where abc and def have different format (e.g., different colours or fonts).
The the sharedString.xml in xlsx have multiple r node in si node:
```xml
<si>
<r>
<t>abc</t>
</r>
<r>
<rPr><sz val="10"/><color rgb="FFFF0000"/><rFont val="微软雅黑"/><charset val="134"/></rPr>
<t>def</t>
</r>
</si>
```
Currently `shared_strings_serializer::read_shared_strings()` only reads the first r node, and should consider as a bug.
This commit fixes this bug by read all `r.t.text` values in one `si` node and concat the text into one single string.
