Update content_length header

When the :update_content_length_header option is set to a truthy value, VCR will ensure that the Content-Length header will have the correct value. This is useful in several situations:

- When you manually edit the cassette file and change the resonse body
  length.  You can use this option so you don't have to manually calculate
  and update the body length.
- When you use ERB, the response body length may vary.  This will ensure
  it is always correct.
- Syck, the default YAML engine for ruby 1.8 (and 1.9, unless you compile
  it to use Psych), has a bug where it sometimes will remove some
  whitespace strings when you serialize them.  This may cause the
  `Content-Length` header to have the wrong value.

This is especially important when you use a client that checks the Content-Length header. Mechanize, for example, will raise an EOFError when the header value does not match the actual body length.

Background

Given a previously recorded cassette file "cassettes/example.yml" with:

--- 
http_interactions: 
- request: 
    method: get
    uri: http://example.com/
    body: 
      encoding: UTF-8
      string: ""
    headers: {}
  response: 
    status: 
      code: 200
      message: OK
    headers: 
      Content-Type: 
      - text/html;charset=utf-8
      Content-Length: 
      - "11"
    body: 
      encoding: UTF-8
      string: Hello <modified>
    http_version: "1.1"
  recorded_at: Tue, 01 Nov 2011 04:58:44 GMT
recorded_with: VCR 2.0.0

And a file named "common_stuff.rb" with:

require 'vcr'

VCR.configure do |c|
  c.cassette_library_dir = 'cassettes'
  c.hook_into :webmock
end

def make_request_and_print_results
  response = Net::HTTP.get_response('example.com', '/')
  puts "Body length: #{response.body.length}"
  puts "Header value: #{response['Content-Length']}"
end

Default :update_content_length_header setting

Given a file named "default.rb" with:

require 'common_stuff'

VCR.use_cassette('example') do
  make_request_and_print_results
end

When I run ruby default.rb

Then the output should contain:

Body length: 16
Header value: 11

:update_content_length_header => false

Given a file named "false.rb" with:

require 'common_stuff'

VCR.use_cassette('example', :update_content_length_header => false) do
  make_request_and_print_results
end

When I run ruby false.rb

Then the output should contain:

Body length: 16
Header value: 11

:update_content_length_header => true

Given a file named "true.rb" with:

require 'common_stuff'

VCR.use_cassette('example', :update_content_length_header => true) do
  make_request_and_print_results
end

When I run ruby true.rb

Then the output should contain:

Body length: 16
Header value: 16