Only draw table header on 1st page if >=1 body row

Check, before rendering the table, to be sure that we can draw the first
body row (excluding any header). If we can't, start the entire table on
the next page.

Closes prawnpdf#152.

lib/prawn/table.rb

@@ -242,9 +242,20 @@ def draw
       # because even if we are in a stretchy bounding box, flowing to the next
       # page will not buy us any space if we are at the top.
       if @pdf.y > @pdf.bounds.height + @pdf.bounds.absolute_bottom - 0.001
+        # we're at the top of our bounds
         started_new_page_at_row = 0
       else
         started_new_page_at_row = -1
+
+        # If there isn't enough room left on the page to fit the first data row
+        # (excluding the header), start the table on the next page.
+        needed_height = row(0).height
+        needed_height += row(1).height if @header
+        if needed_height > @pdf.y - ref_bounds.absolute_bottom
+          @pdf.bounds.move_past_bottom
+          offset = @pdf.y
+          started_new_page_at_row = 0
+        end
       end
 
       @cells.each do |cell|

spec/table_spec.rb

@@ -32,6 +32,10 @@
       }.should.not.raise
     end   
 
+    it "should allow a table with a header but no body" do
+      lambda { @pdf.table([["Header"]], :header => true) }.should.not.raise
+    end
+
     # TODO: pending colspan
     xit "should accurately count columns from data" do
       # First data row may contain colspan which would hide true column count
@@ -469,6 +473,17 @@
       end.page_count.should == 2
     end
 
+    it "should only draw first-page header if the first body row fits" do
+      pdf = Prawn::Document.new
+
+      pdf.y = 60 # not enough room for a table row
+      pdf.table [["Header"], ["Body"]], :header => true
+
+      output = PDF::Inspector::Page.analyze(pdf.render)
+      # Ensure we only drew the header once, on the second page
+      output.pages[0][:strings].should.be.empty
+      output.pages[1][:strings].should == ["Header", "Body"]
+    end
   end
 
   describe "#style" do