copy all array like feature fields after unsafe_wrap (#442)

* `copy` all array like features after `unsafe_wrap`, to prevent GDAL from overwriting the memory

* Revert "`copy` all array like features after `unsafe_wrap`, to prevent GDAL from overwriting the memory"

This reverts commit 3bac8a4.

* copy for specific field types in `fetchfield`

* Update feature.jl

src/ogr/feature.jl

@@ -529,14 +529,22 @@ null, it will return `missing`.
 """
 function getfield(feature::AbstractFeature, i::Integer)
     return if !isfieldset(feature, i)
-        nothing
+        return nothing
     elseif isfieldnull(feature, i)
-        missing
+        return missing
     else
         _fieldtype = getfieldtype(getfielddefn(feature, i))
         try
             _fetchfield = _FETCHFIELD[_fieldtype]
-            _fetchfield(feature, i)
+            if _fieldtype in (OFTIntegerList, OFTRealList, OFTStringList, OFTInteger64List)
+                # copy to ensure that GDAL does not free / overwrite the memory.
+                # the docs for the field fetcher functions mention that the returned 
+                # pointer is not valid for very long.
+                return Base.copy(_fetchfield(feature, i)) 
+            else
+                # for static types, we can just return the returned value.
+                return _fetchfield(feature, i)
+            end
         catch e
             if e isa KeyError
                 error(