Make GRPCHost.callOptions immutable

src/objective-c/GRPCClient/GRPCCall.m

@@ -751,7 +751,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
     GRPCMutableCallOptions *callOptions;
     if ([GRPCHost isHostConfigured:_host]) {
       GRPCHost *hostConfig = [GRPCHost hostWithAddress:_host];
-      callOptions = hostConfig.callOptions;
+      callOptions = [hostConfig.callOptions mutableCopy];
     } else {
       callOptions = [[GRPCMutableCallOptions alloc] init];
     }

src/objective-c/GRPCClient/private/GRPCHost.h

@@ -66,7 +66,7 @@ struct grpc_channel_credentials;
 
 @property(atomic, readwrite) GRPCTransportType transportType;
 
-@property(readonly) GRPCMutableCallOptions *callOptions;
+@property(readonly) GRPCCallOptions *callOptions;
 
 + (BOOL)isHostConfigured:(NSString *)address;