EXPERIMENTAL: do not coerce bytes to str in FitsReader.read_struct.

I think it'd be cleaner for the FITS writer and reader to round-trip
`str` without the Piff object implementations having to do any
coercion, but let's see if it works anyway.

piff/readers.py

@@ -64,11 +64,6 @@ def read_struct(self, name):
         data = self._fits[extname].read()
         assert len(data) == 1
         struct = dict([ (col, data[col][0]) for col in cols ])
-        for k, v in struct.items():
-            # We assume all bytes are supposed to be strs in read_struct, but
-            # not in read_table.
-            if isinstance(v, bytes):
-                struct[k] = v.decode()
         return struct
 
     def read_table(self, name, metadata=None):