Remove unnecessary Redis command for Counter (#130)

* Remove unnecessary Redis command for Counter

* Fix rubocop for README
Broken by: rails/rails@a7ee313

---------

Co-authored-by: Pavel Guseynov <pavel@didlogic.com>

Author: Pavel-Guseynov

README.md

@@ -292,7 +292,7 @@ A development console is available by running `bin/console`.
 
 From there, you can experiment with Kredis. e.g.
 
-```rb
+```erb
 >> str = Kredis.string "mystring"
   Kredis  (0.1ms)  Connected to shared
 =>

lib/kredis/types/counter.rb

@@ -9,14 +9,14 @@ class Kredis::Types::Counter < Kredis::Types::Proxying
 
   def increment(by: 1)
     multi do
-      set 0, ex: expires_in, nx: true
+      set 0, ex: expires_in, nx: true if expires_in
       incrby by
     end[-1]
   end
 
   def decrement(by: 1)
     multi do
-      set 0, ex: expires_in, nx: true
+      set 0, ex: expires_in, nx: true if expires_in
       decrby by
     end[-1]
   end