Piotrmaks arg ptr

[asomers]

Mocking method with String or Vec arguments cause clippy::ptr_arg warning in user code:

#[automock]
trait Foo {
    fn foo(&self, x: String);
}

error: writing `&String` instead of `&str` involves a new object where a slice will do.
 --> mockall/tests/automock_string_arguments.rs:7:1
  |
7 | #[automock]
  | ^^^^^^^^^^^
  |
note: the lint level is defined here
 --> mockall/tests/automock_string_arguments.rs:3:9
  |
3 | #![deny(warnings)]
  |         ^^^^^^^^
  = note: `#[deny(clippy::ptr_arg)]` implied by `#[deny(warnings)]`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

This PR silences the warning.

Fixes #58 .