this isCallback is public & shouldn't have been

Also it makes no sense why it's dangling on the matcher like that to
begin with. It's only needed in a couple places to make sure we're 
looking at a callback matcher before we invoke one.

Author: searls

src/callback.js

@@ -14,7 +14,4 @@ export default _.tap(create({
   // Make callback itself quack like a matcher for its non-invoked use case.
   callback.__name = 'callback'
   callback.__matches = _.isFunction
-
-  callback.isCallback = obj =>
-    obj && (obj === callback || obj.__testdouble_callback === true)
 })

src/matchers/is-callback.js

@@ -0,0 +1,5 @@
+import callback from '../callback'
+
+export default function isCallback (obj) {
+  return obj && (obj === callback || obj.__testdouble_callback === true)
+}

src/store/stubbings.js

@@ -1,6 +1,6 @@
 import _ from '../util/lodash-wrap'
 import argsMatch from '../args-match'
-import callback from '../callback'
+import isCallback from '../matchers/is-callback'
 import config from '../config'
 import log from '../log'
 import store from './index'
@@ -45,9 +45,9 @@ var executePlan = (stubbing, actualArgs, actualContext) => {
 }
 
 var invokeCallbackFor = (stubbing, actualArgs) => {
-  if (_.some(stubbing.args, callback.isCallback)) {
+  if (_.some(stubbing.args, isCallback)) {
     _.each(stubbing.args, (expectedArg, i) => {
-      if (callback.isCallback(expectedArg)) {
+      if (isCallback(expectedArg)) {
         callCallback(stubbing, actualArgs[i], callbackArgs(stubbing, expectedArg))
       }
     })

src/when.js

@@ -1,5 +1,6 @@
 import _ from './util/lodash-wrap'
 import callback from './callback'
+import isCallback from './matchers/is-callback'
 import calls from './store/calls'
 import log from './log'
 import stubbings from './store/stubbings'
@@ -52,7 +53,7 @@ No test double invocation call detected for \`when()\`.
 var concatImpliedCallback = (args, config) => {
   if (config.plan !== 'thenCallback') {
     return args
-  } else if (!_.some(args, callback.isCallback)) {
+  } else if (!_.some(args, isCallback)) {
     return args.concat(callback)
   } else {
     return args