Improve handling of HTTP headers

AnkiConnect.py

@@ -202,16 +202,22 @@ class AjaxServer:
         self.resetHeaders()
 
 
-    def addHeader(self, name, value):
-        self.headers.append([name, value])
+    def setHeader(self, name, value):
+        self.headers[name] = value
 
 
     def resetHeaders(self):
-        self.headers = [
-            ['HTTP/1.1 200 OK', None],
-            ['Content-Type', 'text/json'],
-            ['Content-Length', '']
-        ]
+        self.headers = {
+            'HTTP/1.1 200 OK': None,
+            'Content-Type': 'text/json'
+        }
+
+
+    def getHeaders(self):
+        headers = []
+        for name in self.headers:
+            headers.append([name, self.headers[name]])
+        return headers
 
 
     def advance(self):
@@ -257,8 +263,8 @@ class AjaxServer:
 
         resp = bytes()
 
-        headers = self.headers
-        headers[2][1] = str(len(body))
+        self.setHeader('Content-Length', str(len(body)))
+        headers = self.getHeaders()
 
         for key, value in headers:
             if value is None: