Prohibit setext headers from starting with spaces

Avoids ambiguity between setext headings and in-progress lists (e.g.,
when the point is at X below):

- foo
    - bar
-X

Closes GH-143.

markdown-mode.el

@@ -1277,7 +1277,7 @@ Group 2 matches only the label, without the surrounding markup.
 Group 3 matches the closing square bracket.")
 
 (defconst markdown-regex-header
-  "^\\(?:\\([^\r\n-].*\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
+  "^\\(?:\\([^\r\n- ].*\\)\n\\(?:\\(=+\\)\\|\\(-+\\)\\)\\|\\(#+\\)[ \t]+\\(.*?\\)[ \t]*\\(#*\\)\\)$"
   "Regexp identifying Markdown headings.
 Group 1 matches the text of a setext heading.
 Group 2 matches the underline of a level-1 setext heading.
@@ -1287,7 +1287,7 @@ Group 5 matches the text, without surrounding whitespace, of an atx heading.
 Group 6 matches the closing hash marks of an atx heading.")
 
 (defconst markdown-regex-header-setext
-  "^\\([^\r\n-].*\\)\n\\(=+\\|-+\\)$"
+  "^\\([^\r\n- ].*\\)\n\\(=+\\|-+\\)$"
   "Regular expression for generic setext-style (underline) headers.")
 
 (defconst markdown-regex-header-atx

tests/markdown-test.el

@@ -1455,7 +1455,7 @@ the opening bracket of [^2], and then subsequent functions would kill [^2])."
 
 (ert-deftest test-markdown-complete/setext-header ()
   "Test `markdown-complete' for setext headers."
-  (markdown-test-string " test  \n=="
+  (markdown-test-string "test  \n=="
                         (call-interactively 'markdown-complete)
                         (should (string-equal (buffer-string) "test\n===="))))