Improve Accept and Content-Type handling

Originally, if WSME received an Accept or Content-Type header that
was not aligned with what it was prepared to handle it would error
out with a 500 status code. This is not good behavior for a web
service.

In the process of trying to fix this it was discovered that the
content-negotiation code within WSME (the code that, in part,
looks for a suitable protocol handler for a request) and tests of
that code are incorrect, violating expected HTTP behaviors. GET
requests are passing Content-Type headers to declare the desired
type of representation in the response. This is what Accept is for.

Unfortunately the server-side code was perfectly willing to accept
this behavior. These changes correct that.

Closes-Bug: 1419110
Change-Id: I2b5c0075611490c047b27b1b43b0505fc5534b3b

wsmeext/tests/test_sqlalchemy_controllers.py

@@ -136,12 +136,26 @@ class TestCRUDController():
         DBSession.flush()
         pid = p.id
         r = self.app.get('/person?ref.id=%s' % pid,
-                         headers={'Content-Type': 'application/json'})
+                         headers={'Accept': 'application/json'})
         r = json.loads(r.text)
         print(r)
         assert r['name'] == u('Pierre-Joseph')
         assert r['birthdate'] == u('1809-01-15')
 
+    def test_GET_bad_accept(self):
+        p = DBPerson(
+            name=u('Pierre-Joseph'),
+            birthdate=datetime.date(1809, 1, 15))
+        DBSession.add(p)
+        DBSession.flush()
+        pid = p.id
+        r = self.app.get('/person?ref.id=%s' % pid,
+                         headers={'Accept': 'text/plain'},
+                         status=406)
+        assert r.text == ("Unacceptable Accept type: text/plain not in "
+                          "['application/json', 'text/javascript', "
+                          "'application/javascript', 'text/xml']")
+
     def test_update(self):
         p = DBPerson(
             name=u('Pierre-Joseph'),