spanner: fix lint error

[danielnorberg]

Fix lint error introduced in #332

warning: this expression creates a reference which is immediately dereferenced by the compiler
  --> spanner/src/apiv1/mod.rs:35:49
   |
35 |         cm.conn().with_metadata(client_metadata(&DATABASE))
   |                                                 ^^^^^^^^^ help: change this to: `DATABASE`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
   = note: `#[warn(clippy::needless_borrow)]` on by default

Noting though that this is only in test code. We should perhaps change CI to cargo clippy --all-targets to catch lint errors in test code as well.