tests: make sure echo_app returns proper content type

vcsserver/echo_stub/echo_app.py

@@ -1,34 +1,34 b''
 """
 Implementation of :class:`EchoApp`.
 
 This WSGI application will just echo back the data which it recieves.
 """
 
 import logging
 
 
 log = logging.getLogger(__name__)
 
 
 class EchoApp(object):
 
     def __init__(self, repo_path, repo_name, config):
         self._repo_path = repo_path
         log.info("EchoApp initialized for %s", repo_path)
 
     def __call__(self, environ, start_response):
         log.debug("EchoApp called for %s", self._repo_path)
         log.debug("Content-Length: %s", environ.get('CONTENT_LENGTH'))
         environ['wsgi.input'].read()
         status = '200 OK'
-        headers = []
+        headers = [('Content-Type', 'text/plain')]
         start_response(status, headers)
         return ["ECHO"]
 
 
 def create_app():
     """
     Allows to run this app directly in a WSGI server.
     """
     stub_config = {}
     return EchoApp('stub_path', 'stub_name', stub_config)