Added cache clear

[gnagel]

No description provided.

[featalion]

@gnagel thanks for the PR, @Stephenitis, but it is better to use:

instead

@client.post( "projects/#{ @client.project_id }/caches/#{ opts[:namespace] }/clear" )

it is better to do something like that:

with_namespace('', options) do |cache, _|
  cache.clear
end

[Stephenitis]

I didn't see that existed.
ref:

iron_cache_rails/lib/action_dispatch/session/iron_cache.rb

Lines 52 to 58 in 6ea477c

	def with_namespace(key, options)
	options[:namespace] ||= 'rails_cache'
	cache_name = options[:namespace]
	yield(@client.cache(cache_name), escape_key(key))
	end

[featalion]

do not worry, I haven't tested my solution yet. Anyway, current one works too. But if we will change the IronCache API this method could be broken.