Exclude SSI Comments (#670)

* Exclude SSI comments

Exclude Nginx & Apache Server Side Include (SSI) comments from being stripped via minification, the same way IE directives are not stripped.

Both Nginx & Apache SSI directives begin with:

<!—#

So we can just look for the # and don’t strip the comment in that case.

Nginx SSI: http://nginx.org/en/docs/http/ngx_http_ssi_module.html

Apache SSI: https://httpd.apache.org/docs/2.4/howto/ssi.html

* Add tests for Nginx/Apache SSI directives

Co-authored-by: Andrew Welch <andrew@keluli.local>

tests/_test_files/html/before.html

@@ -9,6 +9,11 @@
 	<meta name="description" content="A demonstration of what can be accomplished visually through CSS-based design." />
 	<meta name="robots" content="all" />
 	<title>css Zen Garden: The Beauty in CSS Design</title>
+	<!--# if expr="$HTTP_COOKIE=/minify\=1/" -->
+	<meta name="generator" content="minify-cookie" />
+	<!--# else -->
+	<meta name="generator" content="minify-no-cookie" />
+	<!--# endif -->
 
 	<!-- to correct the unsightly Flash of Unstyled Content. http://www.bluerobot.com/web/css/fouc.asp -->
 	<script type="text/javascript"><!--
@@ -63,11 +68,11 @@ css hack {
     display:none;
 }
 	</style>
-	<link 
+	<link
 		rel="Shortcut Icon"
 		type="image/x-icon"
 		href="http://www.csszengarden.com/favicon.ico" />
-	<link 
+	<link
 		rel="alternate"
 		type="application/rss+xml"
 		title="RSS"
@@ -78,7 +83,7 @@ css hack {
 <div id="container">
 		<div id="pageHeader">
 			<h1><span>css Zen Garden</span></h1>
-			<h2><span>The Beauty of <acronym title="Cascading Style Sheets">CSS</acronym>  
+			<h2><span>The Beauty of <acronym title="Cascading Style Sheets">CSS</acronym>
 Design</span></h2>
 		</div>
 		<pre>