Add immutable to property descriptions and update pageHideTimeout's

deafult to 0

Fix #8

README.md

@@ -77,13 +77,13 @@ The `PendingBeacon` class would support the following methods/properties:
 
 | *Method/Property Name* | *Description* |
 | ---------------------- | ------------- |
-| `url` | A string property reflecting the target URL endpoint of the pending beacon.  |
-| `method` | A immutable property defining the HTTP method used to send the beacon. Its value is a string matching either `'GET'` or `'POST'`. Defaults to `'POST'`. |
+| `url` | An immutable string property reflecting the target URL endpoint of the pending beacon.  |
+| `method` | An immutable property defining the HTTP method used to send the beacon. Its value is a string matching either `'GET'` or `'POST'`. Defaults to `'POST'`. |
 | `deactivate()` | Deactivate (cancel) the pending beacon. |
 | `setData(data)` | Set the current beacon data. The `data` argument would take the same types as the [sendBeacon][sendBeacon-w3] method’s `data` parameter. That is, one of [`ArrayBuffer`][ArrayBuffer-api], [`ArrayBufferView`][ArrayBufferView-api], [`Blob`][Blob-api], `string`, [`FormData`][FormData-api], or [`URLSearchParams`][URLSearchParams-api]. |
 | `sendNow()` | Send the current beacon data immediately. |
-| `pageHideTimeout` | Defaults to `null`. If set, a timeout in milliseconds after the next `pagehide` event is sent, after which a beacon will be queued for sending, regardless of whether or not the page has been discarded yet. If this is null when the page is hidden, the beacon will be sent on page discard (including eviction from the BFCache). Note that the beacon is not guaranteed to be sent at exactly this many milliseconds after pagehide; bundling/batching of beacons is possible. |
-| `isPending` | A property that returns whether the beacon is still ‘pending’; that is, whether or not the beacon has started the sending process. |
+| `pageHideTimeout` | Defaults to `0`. If set greater than `0`, a timeout in milliseconds after the next `pagehide` event is sent, after which a beacon will be queued for sending, regardless of whether or not the page has been discarded yet. If this is `0` when the page is hidden, the beacon will be sent on page discard (including eviction from the BFCache). Note that the beacon is not guaranteed to be sent at exactly this many milliseconds after pagehide; bundling/batching of beacons is possible. |
+| `isPending` | An immutable property that returns whether the beacon is still ‘pending’; that is, whether or not the beacon has started the sending process. |
 
 Note that attempting to assign a value to any of the properties will have no observable effect.