Don't mutate index name when stripping (#433)

Currently calling `init_index` with a frozen string causes a `FrozenError` as the method tries to mutate `index_name`. Using `strip` instead of `strip!` should fix it.

Author: ronocod

lib/algolia/search_client.rb

@@ -90,11 +90,11 @@ def get_task_status(index_name, task_id, opts = {})
       # @return [Index] new Index instance
       #
       def init_index(index_name)
-        index_name.strip!
-        if index_name.empty?
+        stripped_index_name = index_name.strip
+        if stripped_index_name.empty?
           raise AlgoliaError, 'Please provide a valid index name'
         end
-        Index.new(index_name, @transporter, @config)
+        Index.new(stripped_index_name, @transporter, @config)
       end
 
       # List all indexes of the client