server: error when accessing /exhcange/asset_list

[jfabric]

when the get /exchange/asset_list endpoint is accessed, the following error is emitted on the command line (only first few lines are shown):

events.js:291
throw er; // Unhandled 'error' event
^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
at ServerResponse.setHeader (_http_outgoing.js:543:11)
at ServerResponse.header (/home/jmills/portal/code/CryptoMarketAPI/node_modules/express/lib/response.js:776:10)

reviewer: ?