combine works with NA and character (Closes tidyverse#2203)

This commit fixes tidyverse#2203 by adding compatibility to the character Collecter  so it accepts logical missing values and replaces them with NA_character_ as expected.

inst/include/dplyr/Collecter.h

@@ -109,6 +109,8 @@ namespace dplyr {
         collect_strings(index, v);
       } else if (Rf_inherits(v, "factor")) {
         collect_factor(index, v);
+      } else if (TYPEOF(v) == LGLSXP && all_na(v)) {
+        collect_logicalNA(index, v);
       } else {
         CharacterVector vec(v);
         collect_strings(index, vec);
@@ -120,7 +122,7 @@ namespace dplyr {
     }
 
     inline bool compatible(SEXP x) {
-      return (STRSXP == TYPEOF(x)) || Rf_inherits(x, "factor");
+      return (STRSXP == TYPEOF(x)) || Rf_inherits(x, "factor") || (LGLSXP == TYPEOF(x) && all_na(x));
     }
 
     bool can_promote(SEXP x) const {
@@ -136,6 +138,14 @@ namespace dplyr {
 
   private:
 
+    void collect_logicalNA(const SlicingIndex& index, LogicalVector source) {
+      SEXP* p_data   = Rcpp::internal::r_vector_start<STRSXP>(data);
+      int n = index.size();
+      for (int i=0; i<n; i++) {
+        p_data[index[i]] = NA_STRING;
+      }
+    }
+
     void collect_strings(const SlicingIndex& index, CharacterVector source) {
       SEXP* p_source = Rcpp::internal::r_vector_start<STRSXP>(source);
       SEXP* p_data   = Rcpp::internal::r_vector_start<STRSXP>(data);