header_type attribute of p4_header_stack is a string

[rhalstea]

After building the HLIR the header_type attribute for p4_header_stack objects is a string. I was expecting it to be a p4_header object.

I had a quick look at the code and it seems like the header_type should be updated when the build() method is called, but I don't see where the build() method is ever called. Should https://github.com/p4lang/p4-hlir/blob/p4v1.1/p4_hlir/hlir/p4.py#L103 include hlir.p4_header_stacks?

[antoninbas]

Fixed by df1dd04