firebase-ios-sdk

Fix StringView to properly convert NSStrings (#509)

NSStrings are utf16, so NSString::length returns the number of utf16
characters present in the string. When the string is entirely made up of
US-ASCII characters, everything's fine. But when characters requiring 16
bits are present, the size calculations were incorrect.

Fixed by calculating the length based on the destination character set
(i.e. UTF8).