ATX headers: clarify that a space (or EOL) is needed.

Other whitespace won't do.

Added a test case and modified an existing one to
make this clearer.

Closes #373.

spec.txt

@@ -578,8 +578,8 @@ An [ATX heading](@atx-heading)
 consists of a string of characters, parsed as inline content, between an
 opening sequence of 1--6 unescaped `#` characters and an optional
 closing sequence of any number of unescaped `#` characters.
-The opening sequence of `#` characters cannot be followed directly by a
-[non-whitespace character]. The optional closing sequence of `#`s must be
+The opening sequence of `#` characters must be followed by a
+[space] or by the end of line. The optional closing sequence of `#`s must be
 preceded by a [space] and may be followed by spaces only.  The opening
 `#` character may be indented 0-3 spaces.  The raw contents of the
 heading are stripped of leading and trailing spaces before being parsed
@@ -623,10 +623,18 @@ headings:
 .
 #5 bolt
 
-#foobar
+#hashtag
 .
 <p>#5 bolt</p>
-<p>#foobar</p>
+<p>#hashtag</p>
+.
+
+A tab will not work:
+
+.
+#→foo
+.
+<p># foo</p>
 .
 
 This is not a heading, because the first `#` is escaped: