fix parsing trailing message content from CMGR command

[dragonnn]

I think I have here something for issue #180 that doesn't fell like a huge hack.
Still probably a hack but a hope a small one.
It misuses fields attribute from deserialize_struct(), saves it count in the Deserializer.
Then SeqAccess does count how many fields it already parsed, if it hits the branch that no , are present anymore and the count of parsed fields is exactly one less then the expected count of fields to parse it sets a flag in the Deserializer is_trailing_parsing to indicate that parse_str or parse_bytes should now just take everything left and try to parse it.
This does seam to work fine for my use case, but I bet it still has some edge cases where it might not work.
Doesn't seam to break any other tests and I add a test for my use case.

[MathiasKoch]

I think this looks reasonable..
Perhaps @rmja wants to lend an extra set of eyes here? 👀

[rmja]

I will run it through my sim900 driver test suite and get back.

[rmja]

This change is fine with me, all tests pass fine.