Both symbol keys and string keys won't be deleted on `delete`:

Currently:
```ruby
irb(main):003:0> a = SessionHash.new
=> {}
irb(main):004:0> a.store(:foo, "sym")
=> "sym"
irb(main):005:0> a.store('foo', "string")
=> "string"
irb(main):006:0> a
=> {:foo=>"sym", "foo"=>"string"}
irb(main):007:0> a.delete(:foo)
=> "string"
irb(main):008:0> a
=> {}
```

With code as:
```
        def delete(key)
          super(key.to_s) { super(key) }
        end
```
```ruby
irb(main):002:0> a = SessionHash.new
=> {}
irb(main):003:0> a.store(:foo, "sym")
=> "sym"
irb(main):004:0> a.store('foo', "string")
=> "string"
irb(main):005:0> a
=> {:foo=>"sym", "foo"=>"string"}
irb(main):006:0> a.delete(:foo)
=> "string"
irb(main):007:0> a
=> {:foo=>"sym"}
```