Add Redis#with and Redis#then definitions (#961)

Author: bolshakov

gems/redis/4.2/_test/with.rb

@@ -0,0 +1,6 @@
+redis = Redis.new
+
+redis.with do |conn|
+  conn.set "foo", "bar"
+end
+# => "OK"

gems/redis/4.2/redis.rbs

@@ -276,4 +276,6 @@ class Redis
   # @see #eval
   # @see #evalsha
   def script: (Symbol, *untyped) -> untyped
+
+  def with: [T] () { (self) -> T } -> T
 end