Fix panic when passing in nested objects to dynamic commands

[crufter]

Fixing

        	panic: interface conversion: interface {} is map[string]string, not map[string]interface {}
        
        goroutine 1 [running]:
        github.com/micro/micro/v3/cmd.(*command).Run.func1()
        	/home/crufter/code/src/github.com/micro/micro/cmd/cmd.go:574 +0x136
        panic(0x10b5240, 0xc0005b8db0)
        	/usr/local/go/src/runtime/panic.go:967 +0x15d
        github.com/micro/micro/v3/cmd.flagsToRequest(0xc0005b8d20, 0xc0002c8dc0, 0xc, 0xc00019ac01, 0xc)
        	/home/crufter/code/src/github.com/micro/micro/cmd/dynamic.go:397 +0x824
        github.com/micro/micro/v3/cmd.callService(0xc000593aa0, 0x12a0982, 0x5, 0xc00058e8c0, 0x0, 0x0)
        	/home/crufter/code/src/github.com/micro/micro/cmd/dynamic.go:171 +0x1f6

When passing in multi level object to dynamic commands ie micro users --user_email=email save