bug 59383: getFormat should cache data formatter (fix from Nick C), accidentally deleted in bug 58532 r1710407.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1741114 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/ss/usermodel/DataFormatter.java

@@ -344,6 +344,7 @@ public class DataFormatter implements Observer {
         
         // Build a formatter, and cache it
         format = createFormat(cellValue, formatIndex, formatStr);
+        formats.put(formatStr, format);
         return format;
     }