search: move misc search parsing helpers to PostQueryBuilder.

* Move various search parser helper methods (`has_metatag?`,
  `is_single_tag?` et al) from PostSets and the Tag model to
  PostQueryBuilder.

* Fix various minor bugs stemming from trying to check if a search query
  contains certain metatags using regexes or other adhoc techniques.

test/unit/post_sets/post_test.rb

@@ -65,16 +65,6 @@ module PostSets
         end
       end
 
-      context "a set for the 'a b' tag query" do
-        setup do
-          @set = PostSets::Post.new("a b")
-        end
-
-        should "know it isn't a single tag" do
-          assert(!@set.is_single_tag?)
-        end
-      end
-
       context "a set going to the 1,001st page" do
         setup do
           @set = PostSets::Post.new("a", 1_001)
@@ -118,10 +108,6 @@ module PostSets
           @set = PostSets::Post.new("a")
         end
 
-        should "know it is a single tag" do
-          assert(@set.is_single_tag?)
-        end
-
         should "normalize its tag query" do
           assert_equal("a", @set.tag_string)
         end