Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

It wasn't used for anything interesting and I prefer not having it around to tempt contributors to add new usages of it.

This is built in a way such that its absence from the classpath will not cause failures.

Use get instead of parse for HttpUrl and MediaType where valid input is always required.

Closes: https://github.com/square/retrofit/issues/2425

This avoids a vulnerability on older 2.7 versions.

We probably want to add options to make it strict, or to change the defaults
and make it strict by default and optionally lenient.

We can do that in follow-up. This works.

https://github.com/square/retrofit/issues/2277