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.rb

@@ -1158,7 +1158,7 @@ class RDoc::Markdown
     return _tmp
   end
 
-  # AtxHeading = AtxStart:s @Sp AtxInline+:a (@Sp /#*/ @Sp)? @Newline { RDoc::Markup::Heading.new(s, a.join) }
+  # AtxHeading = AtxStart:s @Spacechar+ AtxInline+:a (@Sp /#*/ @Sp)? @Newline { RDoc::Markup::Heading.new(s, a.join) }
   def _AtxHeading
 
     _save = self.pos
@@ -1169,12 +1169,22 @@ class RDoc::Markdown
         self.pos = _save
         break
       end
-      _tmp = _Sp()
+      _save1 = self.pos
+      _tmp = _Spacechar()
+      if _tmp
+        while true
+          _tmp = _Spacechar()
+          break unless _tmp
+        end
+        _tmp = true
+      else
+        self.pos = _save1
+      end
       unless _tmp
         self.pos = _save
         break
       end
-      _save1 = self.pos
+      _save2 = self.pos
       _ary = []
       _tmp = apply(:_AtxInline)
       if _tmp
@@ -1187,37 +1197,37 @@ class RDoc::Markdown
         _tmp = true
         @result = _ary
       else
-        self.pos = _save1
+        self.pos = _save2
       end
       a = @result
       unless _tmp
         self.pos = _save
         break
       end
-      _save2 = self.pos
-
       _save3 = self.pos
+
+      _save4 = self.pos
       while true # sequence
         _tmp = _Sp()
         unless _tmp
-          self.pos = _save3
+          self.pos = _save4
           break
         end
         _tmp = scan(/\G(?-mix:#*)/)
         unless _tmp
-          self.pos = _save3
+          self.pos = _save4
           break
         end
         _tmp = _Sp()
         unless _tmp
-          self.pos = _save3
+          self.pos = _save4
         end
         break
       end # end sequence
 
       unless _tmp
         _tmp = true
-        self.pos = _save2
+        self.pos = _save3
       end
       unless _tmp
         self.pos = _save
@@ -16539,7 +16549,7 @@ class RDoc::Markdown
   Rules[:_Plain] = rule_info("Plain", "Inlines:a { paragraph a }")
   Rules[:_AtxInline] = rule_info("AtxInline", "!@Newline !(@Sp /\#*/ @Sp @Newline) Inline")
   Rules[:_AtxStart] = rule_info("AtxStart", "< /\\\#{1,6}/ > { text.length }")
-  Rules[:_AtxHeading] = rule_info("AtxHeading", "AtxStart:s @Sp AtxInline+:a (@Sp /\#*/ @Sp)? @Newline { RDoc::Markup::Heading.new(s, a.join) }")
+  Rules[:_AtxHeading] = rule_info("AtxHeading", "AtxStart:s @Spacechar+ AtxInline+:a (@Sp /\#*/ @Sp)? @Newline { RDoc::Markup::Heading.new(s, a.join) }")
   Rules[:_SetextHeading] = rule_info("SetextHeading", "(SetextHeading1 | SetextHeading2)")
   Rules[:_SetextBottom1] = rule_info("SetextBottom1", "/={1,}/ @Newline")
   Rules[:_SetextBottom2] = rule_info("SetextBottom2", "/-{1,}/ @Newline")

test/rdoc/test_rdoc_markdown.rb

@@ -414,10 +414,23 @@ two
   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