Fix image tag with external source

lib/rdoc/markup/to_html.rb

@@ -98,7 +98,10 @@ def handle_RDOCLINK(url) # :nodoc:
 
       gen_url CGI.escapeHTML(url), CGI.escapeHTML(text)
     when /^rdoc-image:/
-      url, alt = $'.split(":", 2) # Split the string after "rdoc-image:" into url and alt
+      # Split the string after "rdoc-image:" into url and alt.
+      #   "path/to/image.jpg:alt text" => ["path/to/image.jpg", "alt text"]
+      #   "http://example.com/path/to/image.jpg:alt text" => ["http://example.com/path/to/image.jpg", "alt text"]
+      url, alt = $'.split(/:(?!\/)/, 2)
       if alt && !alt.empty?
         %[<img src="#{CGI.escapeHTML(url)}" alt="#{CGI.escapeHTML(alt)}">]
       else

test/rdoc/test_rdoc_markup_to_html.rb

@@ -749,6 +749,36 @@ def test_convert_TIDYLINK_image
     assert_not_include result, "<script>"
   end
 
+  def test_convert_TIDYLINK_image_with_alt
+    result =
+      @to.convert '{rdoc-image:path/to/image.jpg:alt text}[http://example.com]'
+
+    expected =
+      "\n<p><a href=\"http://example.com\"><img src=\"path/to/image.jpg\" alt=\"alt text\"></a></p>\n"
+
+    assert_equal expected, result
+  end
+
+  def test_convert_TIDYLINK_image_external
+    result =
+      @to.convert '{rdoc-image:http://example.com/path/to/image.jpg}[http://example.com]'
+
+    expected =
+      "\n<p><a href=\"http://example.com\"><img src=\"http://example.com/path/to/image.jpg\"></a></p>\n"
+
+    assert_equal expected, result
+  end
+
+  def test_convert_TIDYLINK_image_external_with_alt
+    result =
+      @to.convert '{rdoc-image:http://example.com/path/to/image.jpg:alt text}[http://example.com]'
+
+    expected =
+      "\n<p><a href=\"http://example.com\"><img src=\"http://example.com/path/to/image.jpg\" alt=\"alt text\"></a></p>\n"
+
+    assert_equal expected, result
+  end
+
   def test_convert_TIDYLINK_rdoc_label
     result = @to.convert '{foo}[rdoc-label:foottext-1]'