Fix unhandled promise rejection in create monitor

If a monitor was created with an argument that was not a key-value type, a
promise would be rejected outside of the main maxctrl function. Added a
test case that covers this and fixed a few other test coverage problems
that were present.

maxctrl/test/unknown.js

@@ -19,7 +19,8 @@ describe("Unknown Commands", function() {
         'alter',
         'rotate',
         'call',
-        'cluster'
+        'cluster',
+        'drain'
     ]
 
     endpoints.forEach(function (i) {