Make URL part joining aware of absolute URLs.

Previously, with RELATIVE_URLS disabled, when both SITEURL and
STATIC_URL were absolute, the final generate data URLs looked wrong like
this (two absolute URLs joined by `/`):

    http://your.site/http://static.your.site/image.png

With this patch, the data URLs are correctly:

    http://static.your.site/image.png

This also applies to all *_URL configuration options (for example,
ability to have pages and articles on different domains) and behaves
like one expects even with URLs starting with just `//`, thanks to
making use of urllib.parse.urljoin().

However, when RELATIVE_URLS are enabled, urllib.parse.urljoin() doesn't
handle the relative base correctly. In that case, simple os.path.join()
is used. That, however, breaks the above case, but as RELATIVE_URLS are
meant for local development (thus no data scattered across multiple
domains), I don't see any problem.

Just to clarify, this is a fully backwards-compatible change, it only
enables new use cases that were impossible before.

pelican/tests/test_contents.py

@@ -397,6 +397,54 @@ class TestPage(LoggedTestCase):
             '</blockquote>'
         )
 
+    def test_intrasite_link_absolute(self):
+        """Test that absolute URLs are merged properly."""
+
+        args = self.page_kwargs.copy()
+        args['settings'] = get_settings(
+            STATIC_URL='http://static.cool.site/{path}',
+            ARTICLE_URL='http://blog.cool.site/{slug}.html')
+        args['source_path'] = 'content'
+        args['context']['filenames'] = {
+            'images/poster.jpg': Static('',
+                                        settings=args['settings'],
+                                        source_path='images/poster.jpg'),
+            'article.rst': Article('',
+                                   settings=args['settings'],
+                                   metadata={'slug': 'article',
+                                             'title': 'Article'})
+        }
+
+        # Article link will go to blog
+        args['content'] = (
+            '<a href="{filename}article.rst">Article</a>'
+        )
+        content = Page(**args).get_content('http://cool.site')
+        self.assertEqual(
+            content,
+            '<a href="http://blog.cool.site/article.html">Article</a>'
+        )
+
+        # Page link will go to the main site
+        args['content'] = (
+            '<a href="{index}">Index</a>'
+        )
+        content = Page(**args).get_content('http://cool.site')
+        self.assertEqual(
+            content,
+            '<a href="http://cool.site/index.html">Index</a>'
+        )
+
+        # Image link will go to static
+        args['content'] = (
+            '<img src="{filename}/images/poster.jpg"/>'
+        )
+        content = Page(**args).get_content('http://cool.site')
+        self.assertEqual(
+            content,
+            '<img src="http://static.cool.site/images/poster.jpg"/>'
+        )
+
     def test_intrasite_link_markdown_spaces(self):
         # Markdown introduces %20 instead of spaces, this tests that
         # we support markdown doing this.