ensure exec and scalar release connection

ensure exec, query can be executed with Enumerable(Any)
update db doc sample

spec/std/db/db_spec.cr

@@ -69,4 +69,18 @@ describe DB do
       end
     end
   end
+
+  it "exec should return to pool" do
+    with_dummy do |db|
+      db.exec "foo"
+      db.exec "bar"
+    end
+  end
+
+  it "scalar should return to pool" do
+    with_dummy do |db|
+      db.scalar "foo"
+      db.scalar "bar"
+    end
+  end
 end

spec/std/db/statement_spec.cr

@@ -19,6 +19,16 @@ describe DB::Statement do
     end
   end
 
+  it "should initialize positional params in query with array" do
+    with_dummy do |db|
+      stmt = db.prepare("the query")
+      stmt.query ["a", 1, nil]
+      stmt.params[0].should eq("a")
+      stmt.params[1].should eq(1)
+      stmt.params[2].should eq(nil)
+    end
+  end
+
   it "should initialize positional params in exec" do
     with_dummy do |db|
       stmt = db.prepare("the query")
@@ -29,6 +39,16 @@ describe DB::Statement do
     end
   end
 
+  it "should initialize positional params in exec with array" do
+    with_dummy do |db|
+      stmt = db.prepare("the query")
+      stmt.exec ["a", 1, nil]
+      stmt.params[0].should eq("a")
+      stmt.params[1].should eq(1)
+      stmt.params[2].should eq(nil)
+    end
+  end
+
   it "should initialize positional params in scalar" do
     with_dummy do |db|
       stmt = db.prepare("the query")