Implement call method in pipelined block (#283)

Resolves #278

Seems like everything is working; however, I'm not sure if moving
`#call` from `MockRedis` to `MockRedis::Database` has some side-effects
-- is there a reason `call` was separated from other commands?

I also changed `send` to `public_send` in `call` implementation (seems
to be OK, I'm not sure if there are any edge cases when calling private
is a usecase).

Also, I think we should allow mix-cased variants of Redis methods
(`RedisMock.new.call(["GET", 123])` does not work right now, but redis
commands are case insensitive).

Usage example:

```ruby
redis = MockRedis.new

redis.pipelined do |pipeline|
  pipeline.call(["get", "foo"])
  pipeline.call(["get", "bar"])
end
```

Co-authored-by: ya2k <yk@wfolio.com>

lib/mock_redis.rb

@@ -49,10 +49,6 @@ def id
   end
   alias location id
 
-  def call(command, &_block)
-    send(*command)
-  end
-
   def host
     options[:host]
   end

lib/mock_redis/database.rb

@@ -43,6 +43,10 @@ def initialize_copy(_source)
 
     # Redis commands go below this line and above 'private'
 
+    def call(command, &_block)
+      public_send(*command)
+    end
+
     def auth(_)
       'OK'
     end

spec/commands/pipelined_spec.rb

@@ -111,4 +111,26 @@
       expect(results).to eq([value1, value2])
     end
   end
+
+  context 'with `call` method' do
+    let(:key1)   { 'hello' }
+    let(:key2)   { 'world' }
+    let(:value1) { 'foo' }
+    let(:value2) { 'bar' }
+
+    before do
+      @redises.set key1, value1
+      @redises.set key2, value2
+    end
+
+    it 'returns results of pipelined operations' do
+      results = @redises.pipelined do |redis|
+        redis.call(['get', key1])
+        redis.call(['set', key2, 'foobar'])
+        redis.call(['get', key2])
+      end
+
+      expect(results).to eq([value1, 'OK', 'foobar'])
+    end
+  end
 end