Accept a custom reason phrase for the HTTP response (Fixes #25)

src/microdot_asyncio.py

@@ -74,9 +74,10 @@ class Response(BaseResponse):
         self.complete()
 
         # status code
+        reason = self.reason if self.reason is not None else \
+            ('OK' if self.status_code == 200 else 'N/A')
         await stream.awrite('HTTP/1.0 {status_code} {reason}\r\n'.format(
-            status_code=self.status_code,
-            reason='OK' if self.status_code == 200 else 'N/A').encode())
+            status_code=self.status_code, reason=reason).encode())
 
         # headers
         for header, value in self.headers.items():