add headers_only options to csv_builder to only generate headers

lib/as_csv/csv_builder.rb

@@ -26,7 +26,11 @@ def headers
       end
 
       def data_rows
-        @data_rows ||= csv_hashes.collect { |csv_hash| data_row csv_hash }
+        @data_rows ||= if options[:headers_only]
+                         []
+                       else
+                         csv_hashes.collect { |csv_hash| data_row csv_hash }
+                       end
       end
 
       def validate

spec/lib/csv_builder_spec.rb

@@ -21,6 +21,14 @@
       it 'should render correctly' do
         should eq "first,second,third\n1,2,3\n"
       end
+
+      context 'with headers_only option' do
+        let(:options) { { headers_only: true } }
+
+        it 'should render only headers' do
+          should eq "first,second,third\n"
+        end
+      end
     end
 
     context 'with homogenous records' do
@@ -37,9 +45,17 @@
       it 'should render correctly' do
         should == "first,second,third\n10,20,30\n11,21,31\n"
       end
+
+      context 'with headers_only option' do
+        let(:options) { { headers_only: true } }
+
+        it 'should render only headers' do
+          should eq "first,second,third\n"
+        end
+      end
     end
 
-    context 'with hetreogenous records' do
+    context 'with heterogeneous records' do
       let(:records) do
         2.times.map do |i|
           double(:foo, :as_csv => {
@@ -56,6 +72,14 @@
 ,,,11,21,31
 "
       end
+
+      context 'with headers_only option' do
+        let(:options) { { headers_only: true } }
+
+        it 'should render only headers' do
+          should eq "first0,second0,third0,first1,second1,third1\n"
+        end
+      end
     end
 
     context 'with record not respond_to? `as_csv`' do