Consider changing start/endkey behavior

[flimzy]

At present, one must pass a valid JSON object value as a endkey or startkey value (same applies to other related keys). Example:

func ExampleDB_Query() {
	rs := db.Query(context.TODO(), "_design/foo", "_view/bar", kivik.Params(map[string]interface{}{
		"startkey": `"foo"`,                           // Quotes are necessary so the
		"endkey":   `"foo` + kivik.EndKeySuffix + `"`, // key is a valid JSON object
	}))

Consider whether to change this behavior, to simply json-encode whatever value is passed, including a string. This would allow passing foo, which would then be encoded to "foo". We already support passing a json.RawMessage() value, or passing complex values, which get marshaled. So it might be more consistent this way.