iOS: Change default resolution

Hi, just wonder how to change default resolutions. Because it looks like ipad mini cannot handler 640*480 quite well.

I change the constrains to

```
self.constraints = [[RTCMediaConstraints alloc]
initWithMandatoryConstraints:
@[
[[RTCPair alloc] initWithKey:@"OfferToReceiveAudio" value:@"true"],
[[RTCPair alloc] initWithKey:@"OfferToReceiveVideo" value:(self.doVideo ? @"true" : @"false")],
++[[RTCPair alloc] initWithKey:@"maxWidth" value:@"320"],
++[[RTCPair alloc] initWithKey:@"minWidth" value:@"320"],
++[[RTCPair alloc] initWithKey:@"maxHeight" value:@"240"],
++ [[RTCPair alloc] initWithKey:@"minHeight" value:@"240"],
++ [[RTCPair alloc] initWithKey:@"maxFrameRate" value:@"15"],
]
optionalConstraints:
@[
[[RTCPair alloc] initWithKey:@"internalSctpDataChannels" value:@"true"],
[[RTCPair alloc] initWithKey:@"DtlsSrtpKeyAgreement" value:@"true"],
]
];
```

But threw this error: "CreateAnswer called with invalid constraints". I have been googling for a while and still no answer, could you guys help me with this ? Thank you