fix(gorouter): hash_balance as string (partial revert of #535)

[a18e]

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Summary

This PR restores the original parsing of hash_balance.
The ,string suffix was removed as part of https://github.com/cloudfoundry/routing-release/pull/535/changes#diff-a3cb8cde907ca8ef9a573547cc9464f3bfa9348ce3beedcb838e768d40a99c85L46, which broke hash-based routing.

Backward Compatibility

Breaking Change? No, fixes (unintended?) breaking change.

See #585

[b1tamara]

Could you please implement a new unit test in subscriber_test.go to ensure this regression is avoided during NATS message registration.

Something like:

It("parses hash_balance from the incoming JSON string", func() {
				data := []byte(`{"host":"host","app":"app","protocol":"http2","uris":["test.example.com"],"options":{"loadbalancing":"hash","hash_header":"X-Header","hash_balance":"1.5"}}`)

				err := natsClient.Publish("router.register", data)
				Expect(err).ToNot(HaveOccurred())

				Eventually(registry.RegisterCallCount).Should(Equal(1))
				_, originalEndpoint := registry.RegisterArgsForCall(0)
				expectedEndpoint := route.NewEndpoint(&route.EndpointOpts{
					Host:                   "host",
					AppId:                  "app",
					Protocol:               "http2",
					LoadBalancingAlgorithm: "hash",
					HashHeaderName:         "X-Header",
					HashBalanceFactor:      1.5,
				})

				Expect(originalEndpoint).To(Equal(expectedEndpoint))
			})