Fix the command-line CSV importer under Ruby 1.9

Under Ruby 1.8.7, you can parse a CSV file with the
following code (Example A):

  require 'csv'
  CSV.parse('foo.csv') do |row|
      puts "got row: #{row.inspect}"
  end

Rather confusingly, under Ruby 1.8.7, CSV.parse can also
take a string representation of the contents of the file
as its parameter, so this also works (Example B):

  require 'csv'
  CSV.parse("1,hello,red\n2,goodbye,green") do |row|
      puts "got row: #{row.inspect}"
  end

However under Ruby 1.9.3, CSV.parse only expects a string
representation of the contents of the CSV file, so only
Example B works; Example B fails silently (interpreting
the filename as a single cell CSV file, typically).

The import:import_csv rake task unfortunately relied on
both A and B working.  This commit fixes this by adding
PublicBody.import_csv_from_file, and refactoring
PublicBody.import_csv to use the newly added class method,
and adds a test to check for any regression in this
behaviour.

(This means that the usage of import_csv in the admin public
body controller's import_csv action could now be changed
to use PublicBody.import_csv_from_file directly from the
uploaded file, which would be more efficient and cope
with larger files without using lots of memory.)

Fixes #1229