Only parse valid URLs

Only valid characters for URLs should be used for generating URLs.

A list of valid characters can be found in sections 2.2 and 2.3 of IETF
RFC 3986 (https://www.ietf.org/rfc/rfc3986.txt).

lib/rdoc/markup/to_html.rb

@@ -61,12 +61,14 @@ def initialize options, markup = nil
   #
   # These methods are used by regexp handling markup added by RDoc::Markup#add_regexp_handling.
 
+  URL_CHARACTERS_REGEXP_STR = /[A-Za-z0-9\-._~:\/\?#\[\]@!$&'\(\)*+,;%=]/.source
+
   ##
   # Adds regexp handlings.
 
   def init_regexp_handlings
     # external links
-    @markup.add_regexp_handling(/(?:link:|https?:|mailto:|ftp:|irc:|www\.)\S+\w/,
+    @markup.add_regexp_handling(/(?:link:|https?:|mailto:|ftp:|irc:|www\.)#{URL_CHARACTERS_REGEXP_STR}+\w/,
                                 :HYPERLINK)
     init_link_notation_regexp_handlings
   end

test/rdoc/test_rdoc_markup_to_html.rb

@@ -707,6 +707,7 @@ def test_convert_TIDYLINK_irc
   def test_convert_with_exclude_tag
     assert_equal "\n<p><code>aaa</code>[:symbol]</p>\n", @to.convert('+aaa+[:symbol]')
     assert_equal "\n<p><code>aaa[:symbol]</code></p>\n", @to.convert('+aaa[:symbol]+')
+    assert_equal "\n<p><code>https:</code>-foobar</p>\n", @to.convert('<tt>https:</tt>-foobar')
     assert_equal "\n<p><a href=\":symbol\">aaa</a></p>\n", @to.convert('aaa[:symbol]')
   end