Fix incorrect assertion check (#107)

8182446 introduced a regression. Here's
the relevant part of that diff:

```
- assert(isFunction(pageCallback),
-     'The first parameter to `eachPage` must be a function');
+ if (!isFunction(done)) {
+     throw new Error('The first parameter to `eachPage` must be a function');
+ }
```

This should have been checking whether `pageCallback` was a function but
instead changed to `done`.

I re-reviewed the original commit and didn't see any other mistakes.

lib/query.js

@@ -67,7 +67,7 @@ var Query = Class.extend({
      * `done(error)`.
      */
     eachPage: function(pageCallback, done) {
-        if (!isFunction(done)) {
+        if (!isFunction(pageCallback)) {
             throw new Error('The first parameter to `eachPage` must be a function');
         }