[client] Set proper mime-type.

This patch fixes a problem that shows warnings on a browser such as

Resource interpreted as Stylesheet but transferred with MIME type text/html: "http://localhost:8000/test/mocha.css". index.html:4
Resource interpreted as Script but transferred with MIME type text/html: "http://localhost:8000/test/mocha.js". index.html:7
Resource interpreted as Script but transferred with MIME type text/html: "http://localhost:8000/test/expect.js". index.html:8
Resource interpreted as Script but transferred with MIME type text/html: "http://localhost:8000/test/test_hatohol_connector.js". index.html?grep=HatoholMessageBox:18
Resource interpreted as Script but transferred with MIME type text/html: "http://localhost:8000/test/test_hatohol_session_manager.js". index.html?grep=HatoholMessageBox:17