Do not manipulate r-value of assignment

Assignment operation should not change the r-value.

Replacing elements in given arrays will introduce unexpected behaviors like this:

```ruby
new_tags = %w(a b c)
tags.map(&:to_sym)
# => [:a, :b, :c]

org.tags = new_tags

tags
# => [#<ZendeskAPI::Tag {"id"=>"a"}>, #<ZendeskAPI::Tag {"id"=>"b"}>, #<ZendeskAPI::Tag {"id"=>"c"}>]

tags.map(&:to_sym)
# => [nil, nil, nil]
```