Return response time logger token as string to keep '0'

This changes the value returned by the response-time logger token
to be a string instead of a number so that when the response time is
zero, it will not be replaced with a dash (-) in the logs.
Merge pull request #910 from dougwilson/fix/logger-response-time-token-zero

When response time is zero, the logger writes a '-' instead of '0'