Update normalizedFunctionString to handle named functions

The normalizedFunctionString helper needs to be updated to standardize the string representation of named functions during testing. For example, passing in either of the following functions:

function () {}

or

function func () {}

should result in the same string representation:

'function () {}'

This will fix a bug in which there are discrepancies between the results of calling toString in the browser vs. in node, which becomes relevant when testing js-bson in the browser.