Tweaks for CRAN Package Check Results

[trevorld]

• Tweak package or unit tests so we don't get any CRAN check ERRORs: https://cran.r-project.org/web/checks/check_results_xmpdf.html
• [x] Update supports check for "cat_pages_gs()
• [x] Skip Unicode "docinfo" tests on CRAN or implement #57

[trevorld]

I can trigger similar errors if I do LC_ALL=C R CMD check xmpdf_0.1.1.tar.gz locally

[trevorld]

• Currently it appears write support for Unicode input for all three backends requires user in a Unicode locale
• https://kevinushey.github.io/blog/2018/02/21/string-encoding-and-r/