Tests for the derivatives() & related functions failing under R-devel

[gavinsimpson]

This seems to be related to the change to stringsAsFactors = FALSE and is affecting 6 tests in `test-derivatives.R:

E  Running 'test-all.R' [95s] (1m 29.2s)
   Running the tests in 'tests/test-all.R' failed.
   Last 13 lines of output:
      6. gratia:::central_finite_diff2(mod, df)
      8. base::Ops.data.frame(result, h)
      7. gratia:::shift_values(newdata, h = h, i = ind, FUN = "+")
     
     == testthat results  ===========================================================
     [ OK: 770 | SKIPPED: 77 | WARNINGS: 0 | FAILED: 6 ]
     1. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#270) 
     2. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#274) 
     3. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#278) 
     4. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#282) 
     5. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#286) 
     6. Failure: internal finite diff functions fail for all factor vars (@test-derivatives.R#290) 
     
     Error: testthat unit tests failed

[gavinsimpson]

See #61 for further issues related