app: Map HEAD requests to GET

After much discussion, the WSGI community has mostly agreed that
applications should not treat GET and HEAD requests differently. The
HTTP specification practically demands that they are handled the same
way by applications, with the exception that responses to HEAD requests
do not send a body. When they are handled differently, there is an
unfortunate tendency to over-optimize the HEAD request, causing
discrepancies between the headers it returns and those returned by GET.
Further, WSGI middleware may need access to the response body in order
to properly manipulate the response (e.g. to rewrite links, etc.), which
could effect the response to HEAD requests as well.

This commit changes the behavior of `BaseApplication` to change the
method of all `HEAD` requests to `GET` prior to dispatching them.
`Request` objects now have a `real_method` attribute, which indicates
the original method of the request.

http://blog.dscpl.com.au/2009/10/wsgi-issues-with-http-head-requests.html

src/milla/app.py

@@ -75,6 +75,11 @@ class BaseApplication(object):
             allowed_methods = self._find_attr(func, 'allowed_methods')
         except AttributeError:
             allowed_methods = milla.DEFAULT_METHODS
+        if request.method == 'HEAD':
+            request.real_method = 'HEAD'
+            request.method = 'GET'
+        else:
+            request.real_method = request.method
         if request.method not in allowed_methods:
             allow_header = {'Allow': ', '.join(allowed_methods)}
             if request.method == 'OPTIONS':
@@ -99,11 +104,7 @@ class BaseApplication(object):
         if isinstance(response, _string) or not response:
             response = request.ResponseClass(response)
 
-        if environ['REQUEST_METHOD'] == 'HEAD':
+        return response(environ, start_response)
-            start_response(response.status, response.headerlist)
-            return ''
-        else:
-            return response(environ, start_response)
 
     def _call_after(self, func):
         try: