symbol support for namespace

If using syntax like 
```ruby
dc = Dalli::Client.new('foo', :namespace => :symbol_ns)
``` 
and the length of key is more than  250, Dalli will raise error like  
```ruby
#<NoMethodError: undefined method `size' for :symbol_ns:Symbol>
```

This pull request add symbol support for namespace so that there will be no errors raised when using this kind of syntax.

Please let me know if you have any advice.