fix(preheat): fix preheat task log display

Signed-off-by: chlins <chlins.zhang@gmail.com>

api/v2.0/swagger.yaml

@@ -10,6 +10,7 @@ schemes:
 basePath: /api/v2.0
 produces:
   - application/json
+  - text/plain
 consumes:
   - application/json
 securityDefinitions:
@@ -1145,6 +1146,10 @@ paths:
       responses:
         '200':
           description: Get log success
+          headers:
+            Content-Type:
+              description: Content type of response
+              type: string
           schema:
             type: string
         '400':

src/server/v2.0/handler/preheat.go

@@ -679,7 +679,7 @@ func (api *preheatAPI) GetLog(ctx context.Context, params operation.GetLogParams
 		return api.SendError(ctx, err)
 	}
 
-	return operation.NewGetLogOK().WithPayload(string(l))
+	return operation.NewGetLogOK().WithPayload(string(l)).WithContentType("text/plain")
 }
 
 // ListProvidersUnderProject is Get all providers at project level