(MERGE FIRST) Fix check on CI: Use stringsAsFactors = FALSE

[maurolepore]

MERGE this PR first so that the checks pass for all other open PRs.

---

Fixes this error, which surfaces only in some platforms:

-- Failure (test-resample_agb.R:12:3): resample_agb returns a dataframe of size 1e6 x 3 -- res_test$equation_id inherits from factor not character.

https://github.com/forestgeo/allodb/pull/124/checks?check_run_id=1893660473

[coveralls]

Pull Request Test Coverage Report for Build 1889

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.226%

---

Totals
Change from base Build 1878:	0.02%
Covered Lines:	359
Relevant Lines:	381

---

💛 - Coveralls