Routine argument list is not a Dummy_Arg_List if it contains a single argument.

[arporter]

Although we recently (#170) moved to allowing lists to contain 1 item, it seems this change hasn't carried through to the handling of routine arguments. When parsing:

subroutine tmp(a, b)

the resulting parse tree is:

Subroutine_Stmt(None, Name('tmp'), Dummy_Arg_List(',', (Name('a'), Name('b'))), None)

but for:

subroutine tmp(a)

it is:

Subroutine_Stmt(None, Name('tmp'), Name('a'), None)