Bug fix in middleware/authenticate

[fergus99]

-Fix bug in middelware/authenticate where strategy was not copied using Object.create() when passed
into authenticate() as an object.
-add tests to validate that calling authenticate() does not modify the original strategy object

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[fergus99]

There is a bug in middleware/authenticate which causes the strategy passed in to passport.authenticate() to be modified. This can cause unexpected behavior when eg. the success function is modified during a callback. This only occurs when the object is passed into passport.authenticate() directly, not the string name.