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Use proper escaping for proxy passwords

URI.escape does not correctly escape some characters for HTTP proxy
usernames and passwords. This means that passwords containing an "@"
result in an exception.

Now RubyGems uses proper escaping to allow all possible passwords.

Patch by Masahiro Tomita.

Fixes #668

See also ruby bug #8979, @c355880 (original patch adapted for master)
Add extra auth_info escaping tests

These tests are from @c355880 to ensure that the escaping behavior is
not reverted accidentally.

See also #668, ruby bug #8979