Fix encoding for FVInt16 and FVInt64.

According to [4.2.1 Formal Protocol Grammar](http://www.rabbitmq.com/resources/specs/amqp0-9-1.pdf) in the AMQP Protocol Specification, `short-int` and `long-long-int` are encoded as follows:
```
'U' short-int
'L' long-long-int
```
Whereas the currently used `'s'` and `'l'` are used for `short-string` and `long-long-uint` respectively.