capture ruby block in erb and slim templates, fixes #1582

ref #1582 

This patch allows this weird stuff:

ERB:

```ruby
<%= content_tag(:a) { content_tag :b, 'c' } %>
```

Slim:

```ruby
= content_tag(:a) { content_tag :b, 'c' }
```

It was not working before because `__in_erb_template` and `__in_slim_template` flag was set in the preamble for generated ruby code and these flags were affecting behavior of #content_tag called with ruby or template block.

@minad, @padrino/core-members better suggestions would be appreciated