Improve RequestBuffer performance by only encoding when necessary.

While doing some internal benchmarks I noticed that append was taking a fair amount of time. The string duplication can be avoided if we only ensure the encoding is "ASCII-8BIT" when we care about it.

The benchmark was a while loop sending 10,000 messages one at a time. The output is a flat print from ruby-prof.

Master:
```bash
Benchmark: 0.9070836999999999 ms/message

Thread ID: 70308971538160
Fiber ID: 70308972131200
Total: 9.064002
Sort by: self_time

 %self      total      self      wait     child     calls  name
  7.04      1.346     0.639     0.000     0.707   180008   Poseidon::Protocol::RequestBuffer#append
  4.28      0.689     0.388     0.000     0.301   190021   Poseidon::Protocol::ProtocolStruct#type_map
  2.62      0.560     0.238     0.000     0.322   180008   Kernel#dup
  2.58      0.234     0.234     0.000     0.000   380042   Module#===
  2.58      0.234     0.234     0.000     0.000   190007   Array#pack
  2.21      0.322     0.201     0.000     0.121   180008   Kernel#initialize_dup
  2.19      0.198     0.198     0.000     0.000   360036   Kernel#class
  2.18      0.197     0.197     0.000     0.000   190021   <Class::Poseidon::Protocol::ProtocolStruct>#type_map
```

Patch:
```bash
Benchmark: 0.8167268 ms/message

Thread ID: 70131191768800
Fiber ID: 70131192928200
Total: 8.161264
Sort by: self_time

 %self      total      self      wait     child     calls  name
  4.75      0.688     0.388     0.000     0.301   190021   Poseidon::Protocol::ProtocolStruct#type_map
  3.17      0.259     0.259     0.000     0.000   180008   Poseidon::Protocol::RequestBuffer#append
  2.99      0.244     0.244     0.000     0.000   190007   Array#pack
  2.84      0.232     0.232     0.000     0.000   380042   Module#===
  2.42      0.197     0.197     0.000     0.000   360036   Kernel#class
  2.41      0.196     0.196     0.000     0.000   190021   <Class::Poseidon::Protocol::ProtocolStruct>#type_map
```

IMO, the code is less clean than the original but it's contained within `RequestBuffer` and shows a noticeable performance gain.