Accept unicode strings from python functions

When a provider raises Exception with unicode string, such as:
        raise pywbem.CIMError(pywbem.CIM_ERR_FAILED, u"Error message")
A client gets the error message:
lmiwbem_core.CIMError: (1, 'CIM_ERR_FAILED: Python function "invoke_method" didn\'t return a {<int>, <str>) two-tuple')

With the proposed fix the message would be:
lmiwbem_core.CIMError: (1, 'CIM_ERR_FAILED: Error message')