SG panics if _bulk_docs is passed empty JSON object

[ajres]

While trying to repo #1026, found that SG will panic if an empty JSON object is passed to _bulk_docs

curl -H "Content-Type: application/json" -d '{}' -X POST "http://localhost:4985/db/_bulk_docs?revs=true&attachments=true"

2015-08-04T14:55:56.292+01:00 HTTP:  #002: POST /db/_bulk_get?revs=true&attachments=true  (ADMIN)
2015/08/04 14:55:56 http: panic serving 127.0.0.1:55514: interface conversion: interface is nil, not []interface {}
goroutine 50 [running]:
net/http.func·011()
    /usr/local/go/src/net/http/server.go:1130 +0xbb
github.com/couchbase/sync_gateway/rest.func·005(0xc2081bcba0, 0x0, 0x0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/bulk_api.go:307 +0xf1
github.com/couchbase/sync_gateway/rest.(*handler).writeMultipart(0xc20805e1b0, 0x6ff8e0, 0x5, 0xc20827b8b8, 0x0, 0x0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/handler.go:463 +0x38d
github.com/couchbase/sync_gateway/rest.(*handler).handleBulkGet(0xc20805e1b0, 0x0, 0x0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/bulk_api.go:360 +0x1b0
github.com/couchbase/sync_gateway/rest.(*handler).invoke(0xc20805e1b0, 0x997560, 0x0, 0x0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/handler.go:159 +0x4b8
github.com/couchbase/sync_gateway/rest.func·015(0xe37c78, 0xc208228140, 0xc2080e80d0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/handler.go:86 +0x7d
net/http.HandlerFunc.ServeHTTP(0xc208199040, 0xe37c78, 0xc208228140, 0xc2080e80d0)
    /usr/local/go/src/net/http/server.go:1265 +0x41
github.com/gorilla/mux.(*Router).ServeHTTP(0xc2081664b0, 0xe37c78, 0xc208228140, 0xc2080e80d0)
    /Users/andy/Projects/sync_gateway/src/github.com/gorilla/mux/mux.go:86 +0x29e
github.com/couchbase/sync_gateway/rest.func·017(0xe37c78, 0xc208228140, 0xc2080e80d0)
    /Users/andy/Projects/sync_gateway/src/github.com/couchbase/sync_gateway/rest/routing.go:240 +0x32f
net/http.HandlerFunc.ServeHTTP(0xc208236100, 0xe37c78, 0xc208228140, 0xc2080e80d0)
    /usr/local/go/src/net/http/server.go:1265 +0x41
net/http.serverHandler.ServeHTTP(0xc208248000, 0xe37c78, 0xc208228140, 0xc2080e80d0)
    /usr/local/go/src/net/http/server.go:1703 +0x19a
net/http.(*conn).serve(0xc2082521e0)
    /usr/local/go/src/net/http/server.go:1204 +0xb57
created by net/http.(*Server).Serve
    /usr/local/go/src/net/http/server.go:1751 +0x35e