chore: more header cleanup for enabling clang-tidy (#1236)

Author: devjgm

google/cloud/spanner/internal/range_from_pagination.h

@@ -114,7 +114,6 @@ class PaginationRange {
       : request_(std::move(request)),
         next_page_loader_(std::move(loader)),
         get_items_(std::move(get_items)),
-        next_page_token_(),
         on_last_page_(false) {
     current_ = current_page_.begin();
   }

google/cloud/spanner/mutations.h

@@ -107,7 +107,7 @@ class WriteMutationBuilder {
     field.set_table(std::move(table_name));
     field.mutable_columns()->Reserve(static_cast<int>(column_names.size()));
     for (auto& name : column_names) {
-      *field.add_columns() = std::move(name);
+      field.add_columns(std::move(name));
     }
   }
 

google/cloud/spanner/testing/matchers.h

@@ -28,7 +28,7 @@ namespace cloud {
 namespace spanner_testing {
 inline namespace SPANNER_CLIENT_NS {
 
-MATCHER_P(IsProtoEqual, value, "") {
+MATCHER_P(IsProtoEqual, value, "Checks whether protos are equal") {
   std::string delta;
   google::protobuf::util::MessageDifferencer differencer;
   differencer.ReportDifferencesToString(&delta);