js: fix `_mergeMediaConstraints()`
js: fix `_mergeMediaConstraints()`

in webrtc constraints,
`mandatory` constraints is an object, `optional` constraints is an array.

use `Array.concat()` instead `spread operator` for an array merge.
note: webrtc only finds first and ignore the rest if duplicate.

zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc,zxcpoiu/react-native-webrtc