Don't allow empty namespace names

For example, an attribute :class was parsed as having an empty
namespace, with local name "class". It is now parsed as no namespace,
local name ":class". Per the namespace spec at

  https://www.w3.org/TR/xml-names/#iri-use

> The empty string, though it is a legal URI reference, cannot be used
> as a namespace name.

Fixes aantron/lambdasoup#39.

src/namespace.ml

@@ -23,6 +23,8 @@ struct
   let parse qualified_name =
     try
       let colon_index = String.index qualified_name ':' in
+      if colon_index = 0 then
+        raise Not_found;
       let prefix = String.sub qualified_name 0 colon_index in
       let suffix =
         String.sub qualified_name

test/test_html_parser.ml

@@ -426,6 +426,17 @@ let tests = [
        10, 13, S  `End_element;
        10, 13, S  `End_element]);
 
+  ("html.parser.attributes" >:: fun _ ->
+    expect "<div :class='foo'></div>"
+      [ 1,  1, S (start_element "html");
+        1,  1, S (start_element "head");
+        1,  1, S  `End_element;
+        1,  1, S (start_element "body");
+        1,  1, S (`Start_element ((html_ns, "div"), [(("", ":class"), "foo")]));
+        1, 19, S  `End_element;
+        1, 25, S  `End_element;
+        1, 25, S  `End_element]);
+
   ("html.parser.links" >:: fun _ ->
     expect
       {|<a href="foo.com?bar=on&acte=123">foo</a>|}