fix: raise a clear error on blank CSV column headers (#197)

lib/active_admin_import/importer.rb

@@ -129,7 +129,14 @@ def process_file
     end
 
     def prepare_headers
-      headers = self.headers.present? ? self.headers.map(&:to_s) : yield
+      headers = self.headers.present? ? self.headers : yield
+      blank_positions = headers.each_index.select { |i| headers[i].to_s.strip.empty? }
+      unless blank_positions.empty?
+        raise ActiveAdminImport::Exception,
+              "blank column header at column #{blank_positions.map { |i| i + 1 }.join(', ')}"
+      end
+
+      headers = headers.map(&:to_s)
       @headers = Hash[headers.zip(headers.map { |el| el.underscore.gsub(/\s+/, '_') })].with_indifferent_access
       @headers.merge!(options[:headers_rewrites].symbolize_keys.slice(*@headers.symbolize_keys.keys))
       @headers

spec/fixtures/files/authors_blank_header_end.csv

@@ -0,0 +1,3 @@
+Name,Last name,Birthday,
+John,Doe,1986-05-01,x
+Jane,Roe,1988-11-16,y

spec/fixtures/files/authors_blank_header_middle.csv

@@ -0,0 +1,3 @@
+Name,,Last name,Birthday
+John,x,Doe,1986-05-01
+Jane,y,Roe,1988-11-16

spec/fixtures/files/authors_empty_header.csv

@@ -0,0 +1,3 @@
+,Name,Last name,Birthday
+x,John,Doe,1986-05-01
+y,Jane,Roe,1988-11-16

spec/import_spec.rb

@@ -190,6 +190,36 @@ def upload_file!(name, ext = 'csv')
     end
   end
 
+  # Issue #197: a CSV with an empty header cell used to crash with
+  # `undefined method 'underscore' for nil`, wherever the blank sits.
+  context 'with a blank column header' do
+    shared_examples 'a rejected blank header' do |fixture, column|
+      before do
+        add_author_resource
+        visit '/admin/authors/import'
+        upload_file!(fixture)
+      end
+
+      it 'reports a clear error and imports nothing' do
+        expect(page).to have_content "blank column header at column #{column}"
+        expect(page).to have_no_content 'undefined method'
+        expect(Author.count).to eq(0)
+      end
+    end
+
+    context 'at the beginning' do
+      it_behaves_like 'a rejected blank header', :authors_empty_header, 1
+    end
+
+    context 'in the middle' do
+      it_behaves_like 'a rejected blank header', :authors_blank_header_middle, 2
+    end
+
+    context 'at the end' do
+      it_behaves_like 'a rejected blank header', :authors_blank_header_end, 4
+    end
+  end
+
   context 'authors already exist' do
     before do
       Author.create!(id: 1, name: 'Jane', last_name: 'Roe')