Merge pull request jake-codes-at-5-am#64 from jake-codes-at-5-am/XL148

XL148 - fix CT_FormulaCell writer

Author: artem-iron

OpenXmlFormats/Spreadsheet/Sheet.cs

@@ -2701,7 +2701,7 @@ internal void Write(StreamWriter sw, string nodeName)
             XmlHelper.WriteAttribute(sw, "r1", this.r1);
             XmlHelper.WriteAttribute(sw, "r2", this.r2);
             XmlHelper.WriteAttribute(sw, "ca", this.ca, false);
-            if(this.si!=0)
+            if(this.t == ST_CellFormulaType.shared || this.si!=0)
                 XmlHelper.WriteAttribute(sw, "si", this.si, true);
             XmlHelper.WriteAttribute(sw, "bx", this.bx, false);
             if (!string.IsNullOrEmpty(this.valueField))