Allow displayLength option to use an integer value

[jchafik]

displayLength currently only accepts BigDecimal when using an expression (at least in Thymeleaf). Since page size should only be a positive integer, it would make sense to allow integer values.

<table dt:table="true" dt:displayLength="${10}">
...
</table>

Returns the following NPE:

    Caused by: java.lang.NullPointerException
    at com.github.dandelion.datatables.thymeleaf.processor.attr.TableDisplayLengthAttrProcessor.doProcessAttribute(TableDisplayLengthAttrProcessor.java:70)
    at com.github.dandelion.datatables.thymeleaf.processor.AbstractTableAttrProcessor.processAttribute(AbstractTableAttrProcessor.java:70)
    at org.thymeleaf.processor.attr.AbstractAttrProcessor.doProcess(AbstractAttrProcessor.java:87)
    at org.thymeleaf.processor.AbstractProcessor.process(AbstractProcessor.java:212)
    ... 111 more