Extract WFS Store parameter creation to its own function

[geographika]

This pull request moves the WFS request parameter creation out of the loadWfsInternal function in GeoExt.data.store.WfsFeatures and into its own function.
This allows the store logic to be used to download the contents of the store in another outputFormat for example:

        var url = store.url;
        var params = store.createParameters();
        params.outputFormat = 'shapezip'; // change the outputFormat for the current store contents

        // now we can request the same data in the store with a different format (if supported by the WFS server)

        // files can't be downloaded using Ext.Ajax.request (due to browser security)
        // so a hidden form is used with standardSubmit set to true
        // this approach does not allow callbacks to be run on success / failure

        Ext.create('Ext.form.Panel', {
            standardSubmit: true
        }).submit({
            params: params,
            url: url
        });

[coveralls]

Pull Request Test Coverage Report for Build 1105381201

• 8 of 10 (80.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 81.175%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/data/store/WfsFeatures.js	8	10	80.0%

Totals
Change from base Build 961162952:	0.1%
Covered Lines:	1387
Relevant Lines:	1635

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💛 - Coveralls

[geographika]

Thanks for the reviews - merging now.

[chrismayer]

Happy 💯 💯 💯 💯 💯 💯 💯 by the way 😉