always spell +mongo-port+ using the same case

[avodonosov]

cl-mongo has a constant +mongo-port+ which is spelled using different cases in the source code.

;; mongo.lisp

(defconstant +mongo-port+ 27017)
  ;;...
(defvar *mongo-default-port* +MONGO-PORT+ "port for the default connection.")
  ;; ...
(defun switch-default-connection (name &key (host "localhost") (db "test") (port +MONGO-PORT+))
  ;; ...
  )

This should not be a problem in the default reader mode.

But when we recompile all the dependencies, this causes a problem on ECL.
I reported it as an ECL bug: https://sourceforge.net/p/ecls/bugs/279/

Still, it will be nice if cl-mongo had a workaround for ECL, but always using capitalized
variang, i.e. change (defconstant +mongo-port+ 27017) to (defconstant +MONGO-PORT+ 27017)

Doing so will also be friendly to Allegro CL modern mode and other cases when lisp reader does not convert all symbols to a single case.