Fix Post.fast_count("pool:1537") == 0 (fixup 134958d1).

Fixup for a bug in 134958d1. `Post.fast_count("pool:1537")` still
returned zero because `Tag.is_simple_tag?` incorrectly parsed
"pool:1537" as a simple tag, so Post.fast_count still looked for it in
the tags table and found the empty "pool:1537" tag.

test/unit/post_test.rb

@@ -2405,6 +2405,13 @@ class PostTest < ActiveSupport::TestCase
 
           assert_equal(100, Post.fast_count("score:42"))
         end
+
+        should "return the correct cached count for a pool:<id> search" do
+          FactoryGirl.build(:tag, name: "pool:1234", post_count: -100).save(validate: false)
+          Post.set_count_in_cache("pool:1234", 100)
+
+          assert_equal(100, Post.fast_count("pool:1234"))
+        end
       end
 
       context "a multi-tag search" do