Handle redefinable operators as identifier

[aycabta]

For example, operators is highlighting as a operator, but redefineable operators as method is highlighting as a method in GitHub:

def sample_method
end

def +(a, b) # this method name is the same color of above
  a + b # this operator is defferent color from method
end

This Pull Request is fixes the behavior.

When @lex_state is :EXPR_FNAME or :EXPR_DOT, the token is method name. So This Pull Request handles redefineable operators as TkId (identifier) in the case.

[hsbt]

Thanks for an additional unit test. It is reasonable change.