Style aside, this reduces bugs. In this case, the return statement was
indented such that it appeared to be part of the else block, whereas it
was actually executed regardless. This makes the code more robust.

This string previously contained a <CRLF> sequence, when commands were
still writing directly to the socket.

ref nodeftpd#2
ref 22c8d17

Methods without a return statement (including an empty statement)
implicitly return "undefined", limiting their usefullness. By
returning "this" from any object method without another return value, we
allow method calls to be chained together.

In this case, if we can't find or access a file, the distinction is
irrelevant.  The generic error message would hint that the file actually
does exist, but is inaccessible for some other reason (insufficient
permissions, etc.). This behavior could have been leveraged by a
malicious user and should be removed.

Resolve file path more clearly

Previously, "self" was being used to maintain a pointer to the "this"
value outside of the anonymous function. Instead, we use the native
Function.bind(...) method to bind the method to the caller's namespace.

Beyond removing a superfluous variable, this also prevents a user from
mistakenly referencing "this" inside the anonymous function instead of
"self".

This commit tests all branches of the MDTM command. It should accept
requests for existing files and reject requests for invalid files.

Following the bug fixed at 322cf37,
it's time to push an update.