Fix implementation of Lexer.detectable?

[ashmaroli]

This fixes the existing memoization implemented in Lexer.detectable?.
Memoizing with ||= is negated when the computed result equals false. Instead, check if the singleton instance variable is defined and if so, return its value. Otherwise, proceed to compute and store result in the instance variable.

An additional change is replacing methods(false) with singleton_methods(false). For a singleton class, the two calls are equivalent and using the latter expresses intent better.

[pyrmont]

Thanks @ashmaroli!

[jneen]

Good catch, thanks @ashmaroli!