Fix Async::HTTP::WebMockClientWrapper mock interface

[postmodern]

• Changed Async::HTTP::WebMockClientWrapper#initialize to use keyword arguments to match Async::HTTP::Client#initialize
• Also initialize Async::HTTP::WebMockClientWrapper#endpoint (see WebMock causes Async::HTTP::Client#endpoint to always be nil #1060).

[postmodern]

JRuby failure looks unrelated.

[bblimke]

@postmodern Thank you for the PR and for making these changes. I noticed that these modifications to Async::HTTP::Client were made 4 years ago. I'm curious why no one has raised concerns about this issue until now.

[postmodern]

@bblimke you're welcome. I'm guessing because no one probably tests the the Async::HTTP::Client#endpoint value, just that it makes the appropriate requests. In my code I wanted to make sure I was connecting to the correct HTTP endpoint in #initialize, so I needed to access subject.client.endpoint.