add now as a timestamp if file not found

if a timestamp for a give file is not found, add the current time as
timestamp to make the browser always reload the file from the server and
not serve a cached version.

In case a file is requested, that is not in the files array, it will not
be possible to tell the browser not to use a cached version. If the file
has been changed meanwhile, the tests will still run with an old
version, which is unwanted.

It is a valid use-case to not have files in the files array, that should
be served. The files might be served by a proxy server (karma itself
provides the possibility to specify a proxy server for a specific
directory). It might not possible for karma to detect any changes in the
files, served by the proxy. Therefor no caching should be done for these
files and the timestamp should always be attached.
only warn about missing timestamp

this is not as bad, as it seems. Only log a warning, if no timestamp
for a file is found.