Revert "Support show_env param in /headers", reverting postmanlabs#457. Fixed postmanlabs#487.

The docstring typo fixes are retained, along with the additional tests,
modified to revert to the original "headers": { dict format.

The reverted change get_dict('headers') -> get_headers() was not required. The
query-string ?show_env=1 was already effective in allowing env headers to be
returned.

This partially reverts commit 31ffe79.

Author: javabrett

httpbin/core.py

@@ -342,7 +342,7 @@ def view_headers():
         description: The request's headers.
     """
 
-    return jsonify(get_headers())
+    return jsonify(get_dict('headers'))
 
 
 @app.route("/user-agent")

test_httpbin.py

@@ -279,7 +279,7 @@ def test_headers(self):
         }
         response = self.app.get('/headers', headers=headers)
         self.assertEqual(response.status_code, 200)
-        self.assertTrue({'Accept', 'Host', 'User-Agent'}.issubset(set(response.json.keys())))
+        self.assertTrue({'Accept', 'Host', 'User-Agent'}.issubset(set(response.json['headers'].keys())))
         self.assertNotIn('Via', response.json)
 
     def test_headers_show_env(self):
@@ -291,7 +291,7 @@ def test_headers_show_env(self):
         }
         response = self.app.get('/headers?show_env=true', headers=headers)
         self.assertEqual(response.status_code, 200)
-        self.assertTrue({'Accept', 'Host', 'User-Agent', 'Via'}.issubset(set(response.json.keys())))
+        self.assertTrue({'Accept', 'Host', 'User-Agent', 'Via'}.issubset(set(response.json['headers'].keys())))
 
     def test_user_agent(self):
         response = self.app.get(