non-lowercase prefixes in env vars don't work

[rschick]

This change broke non-lowercase prefixes: b6c3d22

If prefix contains upper case characters, env vars won't load. For example the assertion below will fail but it should succeed:

process.env['CONF_key'] = 'value';
var conf = require('rc')('CONF');
assert.equal(conf.key, 'value'); // should succeed, but fails