Auto merge of #1898 - jtgeibel:issue-169, r=smarnach
Auto merge of #1898 - jtgeibel:issue-169, r=smarnach

Check that URLs begin with `http://` or `https://`

This switches to a manual check of the string prefix, because
`Url::parse` may sanitize URLs as they are parsed, making it
difficult to ensure both slashes are present.

Fixes: #169

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io