Change the type of HTTP string properties to AsciiString

Related: #3132

Motivation:

Changing the type of the string properties of HttpVersion and
HttpResponseStatus to AsciiString will give us the performance advantage
when encoding it into the wire.

Modifications:

- Change the type of the following properties to AsciiString:
  - HttpVersion.protocolName()
  - HttpVersion.text()
  - HttpResponseStatus.reasonPhrase()
- Inline their respective encode() methods because they are used only in
  the encoders.
- Fix the test failures incurred by the changes above

Result:

Getting close to the machine