Require space between hash/content in ATX heading (#1140)

While writing some Markdown documentation for Rails, I came across an
interesting case where trying to link to an instance method at the start
of a line would instead parse as an H1 heading:

```markdown
#response_body=
```

Expected:

```html
<a href=""><code>#response_body=</code></a>
```

Actual:

```html
<h1>response_body=</h1>
```

According to the CommonMark spec:

> At least one space or tab is required between the # characters and the
> heading’s contents, unless the heading is empty. Note that many
> implementations currently do not require the space. However, the space
> was required by the original ATX implementation, and it helps prevent
> things like the following from being parsed as headings:
>
> Example 64

So while some implementations do not follow this requirement, I believe
RDoc should because it makes it easy to write text similar to Example 64
(which was used in the new test) and it also enables automatically
linking to instance methods at the start of a line.

lib/rdoc/markdown.kpeg

@@ -530,7 +530,7 @@ AtxInline = !@Newline !(@Sp /#*/ @Sp @Newline) Inline
 AtxStart =  < /\#{1,6}/ >
             { text.length }
 
-AtxHeading = AtxStart:s @Sp AtxInline+:a (@Sp /#*/ @Sp)?  @Newline
+AtxHeading = AtxStart:s @Spacechar+ AtxInline+:a (@Sp /#*/ @Sp)?  @Newline
             { RDoc::Markup::Heading.new(s, a.join) }
 
 SetextHeading = SetextHeading1 | SetextHeading2

test/rdoc/test_rdoc_markdown.rb

@@ -414,10 +414,23 @@ def test_parse_escape
   end
 
   def test_parse_heading_atx
-    doc = parse "# heading\n"
+    # CommonMark Example 62
+    (1..6).each do |level|
+      doc = parse "#{"#" * level} heading\n"
+
+      expected = @RM::Document.new(
+        @RM::Heading.new(level, "heading"))
+
+      assert_equal expected, doc
+    end
+
+    # CommonMark Example 64
+    doc = parse "#5 bolt\n\n#hashtag\n"
 
     expected = @RM::Document.new(
-      @RM::Heading.new(1, "heading"))
+      para("#5 bolt"),
+      para("#hashtag"),
+    )
 
     assert_equal expected, doc
   end