Engine: Support <% =begin%> block comments with no space

[marcoroth]

This pull request updates the herb_extract_ruby_to_buffer_with_semicolons method, so we always put a whitespace before the semicolon when extracting Ruby code from a HTML+ERB template.

With that, we can support parsing, compiling, and rendering the following template:

<%
=begin%>
  This, while unusual, is a legal form of commenting.
<%
=end%>
<div>Hey there</div>

The problem is that =begin; isn't valid Ruby syntax

=begin;


=end;
 

Whereas this is:

=begin ;


=end ;
 

So we always replace the closing %> ERB tag with " ;" instead of "; " so it always ends up with valid extracted Ruby code.

Addresses #562 (comment)

/cc @asilano