Fix warnings

I like to run my tests with warnings enabled. In doing so I encountered a couple warnings in FakeRedis. They seemed to be easy fixes.

``` sh
$ git rev-parse --abbrev-ref HEAD
master
$ bundle exec rspec --warnings >/dev/null
.../fakeredis/lib/redis/connection/memory.rb:407: warning: shadowing outer local variable - value
.../fakeredis/lib/fakeredis/sort_method.rb:23: warning: assigned but unused variable - result
# ... a bunch of spec warnings ...
```

``` sh
$ git rev-parse --abbrev-ref HEAD
fix-warnings                                                                                         
$ bundle exec rspec --warnings >/dev/null
# ... a bunch of spec warnings ...
```