Reset handler in TestHelpersCDNServeMuxHandlers

Reset the initial handler in TestHelpersCDNServeMuxHandlers() so that we don't
get the handler of another test. We were originally guarded against it
happening because the test occurred at the very top of the single file, but
after refactoring to separate files it now appears somewhere in the middle.

This caused it to panic when run after the TestNoCacheHeaderSetCookie() test
because an old handler was trying to access a variable that was either no
longer in scope or was using a key that was out of range. Producing this
error:

    ➜  cdn-acceptance-tests git:(master) go test -test.v -edgeHost 172.16.20.10 -skipVerifyTLS -run 'Helper|SetCook'
    2014/06/11 16:31:01 Confirming that CDN is healthy
    === RUN TestNoCacheHeaderSetCookie
    --- PASS: TestNoCacheHeaderSetCookie (0.00 seconds)
    === RUN TestHelpersCDNServeMuxHandlers
    2014/06/11 16:31:01 http: panic serving 127.0.0.1:57297: runtime error: index out of range
    goroutine 16 [running]:
    net/http.func·009()
            /opt/boxen/homebrew/Cellar/go/1.2.1/libexec/src/pkg/net/http/server.go:1093 +0xae
    runtime.panic(0x246dc0, 0x52be17)
            /opt/boxen/homebrew/Cellar/go/1.2.1/libexec/src/pkg/runtime/panic.c:248 +0x106
    _/Users/dcarley/projects/govuk/cdn-acceptance-tests.func·009(0x62bcf0, 0xc2100108c0, 0xc210059d00)
            /Users/dcarley/projects/govuk/cdn-acceptance-tests/cdn_acceptance_test.go:234 +0xf7
    _/Users/dcarley/projects/govuk/cdn-acceptance-tests.(*CDNServeMux).ServeHTTP(0xc2100386e0, 0x62bcf0, 0xc2100108c0, 0xc210059d00)
            /Users/dcarley/projects/govuk/cdn-acceptance-tests/helpers.go:23 +0x12b
    net/http.serverHandler.ServeHTTP(0xc210020460, 0x62bcf0, 0xc2100108c0, 0xc210059d00)
            /opt/boxen/homebrew/Cellar/go/1.2.1/libexec/src/pkg/net/http/server.go:1597 +0x16e
    net/http.(*conn).serve(0xc210058f00)
            /opt/boxen/homebrew/Cellar/go/1.2.1/libexec/src/pkg/net/http/server.go:1167 +0x7b7
    created by net/http.(*Server).Serve
            /opt/boxen/homebrew/Cellar/go/1.2.1/libexec/src/pkg/net/http/server.go:1644 +0x28b
    --- FAIL: TestHelpersCDNServeMuxHandlers (0.00 seconds)
            helpers_test.go:17: EOF
    === RUN TestHelpersCDNServeMuxProbes
    --- PASS: TestHelpersCDNServeMuxProbes (0.00 seconds)
    FAIL
Merge pull request #25 from alphagov/reset_handler_for_helper_test

Reset handler in TestHelpersCDNServeMuxHandlers