Map char[N] to str in Python

The IDL-Python type conversion has various special cases, defined [here](http://design.ros2.org/articles/idl_interface_definition.html), such as mapping `octet[N]` to `bytes`. However it seems like one obvious special case is missing: `char[N]` to `bytes`. A quick test on Galactic shows that this type is currently converted to a NumPy array. Is there any reason for this or is it an oversight?