Retrieving properties of a wiki configuration source can cause a ClassCastException if a previous call cached a diferent data type

The problem was introduced by XWIKI-11163, since a document configuration source caches the first converted value it is requested. When a different data type is requested for the same cache key, the ClassCastException occurs because the cached value can not be statically cast to the newly requested data type.

See more details in the commit.