JSON::State is not using the parameter space_before

The parameter space_before, documented as a string that is put before a : pair delimiter, is never used. 

Script to reproduce the issue:

```ruby
require 'json'
state = JSON::Ext::Generator::State.new( space_before: "bef_delim", space: "aft_delim")
puts JSON.generate({'1' => 2},state)
```
Ruby tracker 
https://bugs.ruby-lang.org/issues/10317