[svn-r26695] Purpose: Fixed HDFFV-7947 (cont.)

Description:
    - Put back the UNUSED parameters in dsets test because the change to remove
      the warning last time caused failure in setting filter, in turn, caused
      failure in the test with such obscure/unrelated errors!
    - Added incRefCount() to other constructors that missed from last time.
Platforms tested:
    Linux/64 (platypus)
    Linux/32 2.6 (jam)
    SunOS 5.11 (emu)

c++/src/H5DataSet.cpp

@@ -63,6 +63,7 @@ DataSet::DataSet() : H5Object(), AbstractDs(), id(H5I_INVALID_HID) {}
 DataSet::DataSet(const hid_t existing_id) : H5Object(), AbstractDs()
 {
     id = existing_id;
+    incRefCount(); // increment number of references to this id
 }
 
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