plugins: use the get method on the Munin::Plugin::HTTP instance instead of request.

This allows to get rid of most references to HTTP::Request (with the
exception of hp2000_), as no extra headers are usually passed.

The Apache plugins are left untouched because a replacement for them
is already standing by.
Merge pull request #51 from Flameeyes/http-plugins-simplify

plugins: use the get method on the Munin::Plugin::HTTP instance instead ...