Get encoding once, use repeatedly

Originally, the application searched for an encoding object each time it
generated a plain-text-mode response. This commit makes the application
find the text encoding object once during startup, assign it to a
constant, and then use that constant when generating plain-text-mode
responses. This is more efficient and has the added benefit that if the
encoding cannot be found, this fact will be discovered when the
application starts up, not when the first plain-text-mode request is
handled.