Elsewhere we `to_sym` the message arg rather than `to_s` it.  While it's usually a bad idea to `to_sym` user input like this (as that's a potential DOS vector), it doesn't worry me here for a few reasons:

* rspec-mocks has _always_ done this, as far as I know, and we've never heard any complaints.
* The user input here is from a programmer, not an end user of their program.  If they want to DOS their own test suite, there's nothing we can do about that.
* I believe ruby internally converts it to a symbol (consider that `methods`, `instance_methods`, etc all return a list of symbols)

I've always heard that `Symbol#== Symbol` is faster than `String#== String` so there's a nice perf win by using symbols.