Use task variable name when task name is undefined.

In fabric.main.extract_tasks(), the dict mapping names to new-style task
objects now uses the variable name of the object when the object's name
attribute is not defined.  (Technically, when not obj.name or obj.name
== 'undefined').

This change brings the behavior for mapping tasks to names in compliance
with the fabric documentation.
http://docs.fabfile.org/en/1.4.3/usage/tasks.html#new-style-tasks:

    Subclass Task (Task itself is intended to be abstract), define a run
    method, and instantiate your subclass at module level.  Instances’
    name attributes are used as the task name; if omitted the instance’s
    variable name will be used instead.

A test case was added to confirm that the task name equal the variable
name when the task object name attribute is undefined.  This test fails
with the old code and passes with the patched code.

fabric/main.py

@@ -210,8 +210,11 @@ def extract_tasks(imported_vars):
         name, obj = tup
         if is_task_object(obj):
             state.env.new_style_tasks = True
-            # Honor instance.name
-            new_style_tasks[obj.name] = obj
+            # Use instance.name if defined
+            if obj.name and obj.name != 'undefined':
+                new_style_tasks[obj.name] = obj
+            else:
+                new_style_tasks[name] = obj
             # Handle aliasing
             if obj.aliases is not None:
                 for alias in obj.aliases:

tests/support/classbased_task_fabfile.py

@@ -0,0 +1,7 @@
+from fabric import tasks
+
+class ClassBasedTask(tasks.Task):
+    def run(self, *args, **kwargs):
+        pass
+
+foo = ClassBasedTask()

tests/test_main.py

@@ -444,6 +444,18 @@ def test_class_based_tasks_are_found_with_proper_name(self):
             eq_(len(funcs), 1)
             ok_('foo' in funcs)
 
+    def test_class_based_tasks_are_found_with_variable_name(self):
+        """
+        A new-style tasks with undefined name attribute should use the instance
+        variable name.
+        """
+        module = fabfile('classbased_task_fabfile.py')
+        from fabric.state import env
+        with path_prefix(module):
+            docs, funcs = load_fabfile(module)
+            eq_(len(funcs), 1)
+            ok_('foo' in funcs)
+
     def test_recursion_steps_into_nontask_modules(self):
         """
         Recursive loading will continue through modules with no tasks