Motivation:
An instance is always equal to itself. It makes sense to skip processing for this case, which isn't uncommon since `AsciiString` is often memoized within an application when used as HTTP header names.
Modification:
`contentEquals` methods first check for instance equality before doing processing.
Result:
`contentEquals` will be faster when comparing an instance with itself.
I couldn't find any unit tests for these methods, only the static version. Let me know if I should add something to `AsciiStringCharacterTest`.
Came up here:
https://github.com/line/armeria/pull/1731#discussion_r280396280
b45a1cd15b