esim/http_json_api.py: support text/plain response Content-Type

Allow returning text/plain Content-Types as 'data' output argument.

So far, all the esim/http_json_api functions require a JSON response.
However, a specific vendor has a list function where the request is JSON
but the response is text/plain CSV data. Allow and return in a dict.

Change-Id: Iba6e4cef1048b376050a435a900c0f395655a790

pySim/esim/http_json_api.py

@@ -256,5 +256,10 @@ class JsonHttpApiFunction(abc.ABC):
             raise HttpHeaderError(response)
 
         if response.content:
+            if response.headers.get('Content-Type').startswith('application/json'):
                 return self.decode(response.json())
+            elif response.headers.get('Content-Type').startswith('text/plain;charset=UTF-8'):
+                return { 'data': response.content.decode('utf-8') }
+            raise HttpHeaderError(f'unimplemented response Content-Type: {response.headers=!r}')
+
         return None