fix label with block in erb

[lest]

The following template code:

<%= label 'post', 'message' do %>
  Message
  <%= text_field 'post', 'message' %>
<% end %>

Generates:

  Message
  <input id="post_message" name="post[message]" size="30" type="text" />
<label for="post_message">
  Message
  <input id="post_message" name="post[message]" size="30" type="text" />
</label>

It happens because of InstanceTag#to_label_tag invokes capture helper (through label_tag and content_tag) and capture helper can't capture output of block because block is in (ActionView::Base context) and capture helper executes in (InstanceTag context).

I've put call label_tag from @template_object when block is passed. Also Helpers::CaptureHelper and Context modules aren't longer needed in InstanceTag.

It could also could be reason for #3385

/cc @josevalim