update the way expectations.go initializes the margs variable

[sivchari]

Hi, This is a small change, but it will send a PR.
In the String function of expectations.go, there is a place where it initializes a variable called margs.
I thought this would be better for Go's slice specification, since the capacity of the slice to be appended can be allocated in advance by the length of the args.

[l3pp4rd]

Hi, in this program it does not matter at all.
It will not be called millions of times in unit tests and it will have no performance impact. I will not merge this for this very reason, hope it will be a lesson for you

[sivchari]

I comprehended you said. Thanks for reply.
If there are something , I'll sent PR again.