ensure the connection is released after exec

avoid releasing connection twice on exec.
avoid releasing connection when QueryMethod#query is blockless called.

src/db/query_methods.cr

@@ -35,7 +35,7 @@ module DB
     # end
     # ```
     def query(query, *args)
-      prepare query, &.query(*args)
+      prepare(query).query(*args)
     end
 
     # Executes a *query* and yields a `ResultSet` with the results.
@@ -200,23 +200,13 @@ module DB
 
     # Performs the `query` and returns an `ExecResult`
     def exec(query, *args)
-      prepare query, &.exec(*args)
+      prepare(query).exec(*args)
     end
 
     # Performs the `query` and returns a single scalar value
     # puts db.scalar("SELECT MAX(name)").as(String) # => (a String)
     def scalar(query, *args)
-      prepare query, &.scalar(*args)
-    end
-
-    private def prepare(query)
-      stm = prepare(query)
-      begin
-        yield stm
-      rescue ex
-        stm.release_connection
-        raise ex
-      end
+      prepare(query).scalar(*args)
     end
   end
 end

src/db/statement.cr

@@ -74,9 +74,9 @@ module DB
     end
 
     private def perform_exec_and_release(args : Enumerable) : ExecResult
-      res = perform_exec(args)
+      return perform_exec(args)
+    ensure
       release_connection
-      res
     end
 
     protected abstract def perform_query(args : Enumerable) : ResultSet