fixing small bug in the new #fetch method

spec/spec_helper.cr

@@ -28,10 +28,11 @@ macro behaves_like_store(store_definition)
     store["key1"].should eq "abc"
 
     # fetch
-    store.fetch("key9") do
+    value = store.fetch("key9") do
       "value9"
     end
     store["key9"].should eq "value9"
+    value.should eq "value9"
 
     # clear
     store.clear.should eq store

src/kiwi/store.cr

@@ -8,11 +8,11 @@ module Kiwi
     abstract def clear : Store
 
     def fetch(key : String, &) : String?
-      get(key).tap do |value|
-        if !value && (value = yield)
-          set(key, value)
-        end
+      value = get(key)
+      if !value && (value = yield)
+        set(key, value)
       end
+      value
     end
 
     def []=(*args)