add some mock tests for http downloads
allow using LWP::UserAgent as HTTP protocol_class

1) Use eval to load the protocol_class requested
2) Add check_http_response to check for HTTP::Response objects

This also adds support for file:// URLs, to allow LWP to be tested
directly.
Test::More 0.94 is required for subtest
Merge pull request #24 from preaction/lwp

Add support for LWP as an HTTP protocol_class