Flatten tests in annotation_builder_spec.rb (#51)

The `model_annotator/annotation_builder_spec.rb` contained a LOT of the
tests from the original gem. This PR attempts to flatten the original
tests and make it easier to understand. After this, it should be easier
to extract tests into their proper places.

Author: drwl

lib/annotate_rb/model_annotator/annotation_builder.rb

@@ -19,7 +19,7 @@ def initialize(klass, options)
       end
 
       def build
-        @info = "# #{header}\n"
+        @info = "#{header}\n"
         @info += schema_header_text
 
         max_size = @model.max_schema_info_width
@@ -51,6 +51,7 @@ def build
 
       def header
         header = @options[:format_markdown] ? PREFIX_MD.dup : PREFIX.dup
+        header = "# #{header}"
         version = begin
           ActiveRecord::Migrator.current_version
         rescue