Parse double for X-Reset-After instead of float, needs more precision

Float did not contain enough precision to store the millisecond unix
time value, which resulted in the second and millisecond values being
slightly off.

This can be easily tested using:

```cs
> DateTimeOffset.FromUnixTimeMilliseconds((long)(1470173022.123f *
1000)).Millisecond
160 // wrong

> DateTimeOffset.FromUnixTimeMilliseconds((long)(1470173022.123 *
1000)).Millisecond
123 // correct
```

src/Discord.Net.Rest/Net/RateLimitInfo.cs

@@ -21,7 +21,7 @@ namespace Discord.Net
             Remaining = headers.TryGetValue("X-RateLimit-Remaining", out temp) && 
                 int.TryParse(temp, out var remaining) ? remaining : (int?)null;
             Reset = headers.TryGetValue("X-RateLimit-Reset", out temp) && 
-                float.TryParse(temp, out var reset) ? DateTimeOffset.FromUnixTimeMilliseconds((long)(reset * 1000)) : (DateTimeOffset?)null;
+                double.TryParse(temp, out var reset) ? DateTimeOffset.FromUnixTimeMilliseconds((long)(reset * 1000)) : (DateTimeOffset?)null;
             RetryAfter = headers.TryGetValue("Retry-After", out temp) &&
                 int.TryParse(temp, out var retryAfter) ? retryAfter : (int?)null;
             Lag = headers.TryGetValue("Date", out temp) &&