Fix duplicate query string params when hashbang is enabled

[jasonwoan]

Problem

When hashbang is enabled, page.start() will re-add location.search params to the hash. This results in duplicate query params when the page is refreshed.

See #525 and #412

Proposed Fix

Add location.search params only when a query string has not been added to the hash.

Notes

It was a challenge to test this. I couldn't figure out how to supply a query string to the iframe window. I ended up configuring jsdom with a query string and testing against window.location. Please advise if there is a better approach.

[coveralls]

Coverage increased (+0.4%) to 93.443% when pulling 5ce2222 on jasonwoan:fix/duplicate-qs into 4f99916 on visionmedia:master.