null for untouched checboxes

ref #542

inst/test-data/specific-redcapr/read-batch-simple/blank-for-gray-true.R

@@ -17,10 +17,10 @@ NA), dob = structure(c(12294, 12121, -13051, -6269, -5375, NA
     "Had a hand for trouble and a eye for cash\n\nHe had a gold watch chain and a black mustache", 
     NA), mugshot = c("mugshot-1.jpg", "mugshot-2.jpg", "mugshot-3.jpg", 
     "mugshot-4.jpg", "mugshot-5.jpg", NA), health_complete = c(1, 
-    0, 2, 2, 0, 0), race___1 = c(0, 0, 0, 0, 1, 0), race___2 = c(0, 
+    0, 2, 2, 0, NA), race___1 = c(0, 0, 0, 0, 1, 0), race___2 = c(0, 
     0, 0, 1, 0, 0), race___3 = c(0, 1, 0, 0, 0, 0), race___4 = c(0, 
     0, 1, 0, 0, 0), race___5 = c(1, 1, 1, 1, 0, 0), race___6 = c(0, 
     0, 0, 0, 1, 0), ethnicity = c(1, 1, 0, 1, 2, NA), interpreter_needed = c(0, 
     0, 1, NA, 0, NA), race_and_ethnicity_complete = c(2, 0, 2, 
-    2, 2, 0)), row.names = c(NA, -6L), class = c("spec_tbl_df", 
+    2, 2, NA)), row.names = c(NA, -6L), class = c("spec_tbl_df", 
 "tbl_df", "tbl", "data.frame"))